In the Book of the Old Tang Dynasty, the first-hand historical materials of the Tang Dynasty are preserved. The Tang Dynasty has a history of 300 years, and wars are frequent. The original historical materials have suffered several disasters, especially the Anshi Rebellion and the military and political scuffle at the end of the Tang Dynasty, resulting in "the legacy of the times and the simplicity of the feet." Fortunately, the National History compiled by Shu Wei in the Su Zong period became the main basis for the compilation of Tang Shi in the later Jin Dynasty. After Su Zong, less historical materials were kept. Especially in the five dynasties of Xuan, Yi, Nuo, Zhao and Ai, even the "true record" was not preserved, just because Zhang Zhaoyuan and others mixed all kinds of rumors and novels in "troubled times", which slightly supplemented the historical facts. When the Book of the Old Tang Dynasty was compiled, it was only more than 30 years before the death of the Tang Dynasty, and many historical materials were directly obtained from the Tang people. Old Tang Shu preserved these precious historical materials. This is one of its great benefits. For example, some sentences in some biographies, such as "practicing today", "enthronement" and "the beginning of enthronement", are obviously traces recorded directly from the national history and records at that time. Only by directly recording the records of people at that time, historical materials are particularly precious. Some biographies also use "lines" and "family biographies" written by people at that time in the Tang Dynasty. These family heirlooms are often written by famous writers in the Tang Dynasty, which not only keep historical materials truthfully, but also have their own characteristics in writing. Zhao Yi's Notes on the Twenty-two Histories commented that the Biography of Feng Changqing, a Book of the Old Tang Dynasty, was "sorrowful for Yu Bo" and "still exists for thousands of years". The Biography of Guo Ziyi written by Pei Ji is "neat from beginning to end, without fabrication, which shows that historians in the Tang Dynasty are older than literature". In the biographies of the Book of the Old Tang Dynasty, the imperial edicts, calligraphy and memorials of the monarch and ministers in the Tang Dynasty were also quoted in large sections. Some of these original materials have disappeared since then, but they have been preserved because of the inclusion of old Tang books. It is precisely because of these advantages that Sima Guang and others used a lot of materials from Sui and Tang Dynasties when compiling Zi Tong Zhi Jian. On the other hand, the Tang Dynasty was a powerful dynasty in China, and a multi-ethnic unified country developed. The Book of Old Tang Dynasty records a large number of historical materials of China ethnic minorities and their close relationship with the Tang Dynasty in the Central Plains. For example, the record of the marriage between Princess Wencheng and Songzan Gampo, the historical record of Princess Jincheng's entry into Tibet, and the historical records of many ethnic groups in the north, northwest, northeast and southwest such as Turkic, Uighur, Tubo and Qidan are all recorded in the Book of Old Tang Dynasty. In the history of the relationship between the Tang Dynasty and its neighbors Japan, Korea and India, the Book of the Tang Dynasty is also recorded in detail. Its weight is much more than other similar official histories, and the historical facts are more reliable.
The New Tang Book also has obvious shortcomings, and the most important point is that feudal orthodoxy is more serious. Writers criticized the peasant uprising in the late Sui and Tang Dynasties. The Biography of Huang Chao is preceded by the word "rebellion": at the end of Sui Dynasty, Dou Jiande and other peasant troops used extremely vicious words such as "courageously protecting Mao", "gnashing one's teeth and shaking poison" and "evil spirits are blazing". Wu Zetian was falsely accused of "killing the king and usurping the country", and claimed that the purpose of writing "Biography of Wuhou" was to "write his great evil" for liquidation and so on. And so on, it can be seen that the new Tang book is superior to the old Tang book in orthodoxy. As far as writing is concerned, the new books in the Tang Dynasty are not as good as the old ones in the Tang Dynasty. For example, some subjects are too simple, and even unreasonably deleted. Wang Mingsheng, a historical critic in Qing Dynasty, once mentioned in On the Seventeen Histories that the edition of New Tang Book is nearly 67% less than that of old books. According to some statistics, there are nearly 300,000 words in Old Tang Book, only 90,000 words in New Tang Book, about 1.3 million words in Old Book and about 1000 words in New Book. This simplified way of writing makes the Book of the New Tang Dynasty lose many important historical materials. Also, Song Qi and Ouyang Xiu did not hesitate to delete many important plots, such as Biography of Feng Changqing and Biography of Gao Xianzhi, which were very vivid and tragic in Old Tang Book until the new book was cut. In particular, due to the prejudice of rejecting Buddhism, Xuanzang and his party were not included in the New Tang Book, which led to the failure of these two great translators and scientists to be reflected in the New Tang Book.
Although Zi Jian is known as "rigorous writing, clear context, grand trap, thoughtful text, rich historical materials, detailed textual research, detailed narration and proper brevity", there are also many mistakes in Zi Jian itself, such as historian Yan Gengwang's pointing out that Zi Jian has a wrong arrangement order for the 28 generals of Yuntai; The second book of Tang Dynasty mistook "secret service" for "cashier", but "A Mirror with Resources" was not corrected, so it was wrong. Gao's Confucian Qi thinks that Zi and Jian will only see the official Yang Guifei and unofficial history's An Lushan, and lose objectivity. Another example is Yu Hongzhi, the eunuch of the Tang Dynasty. The Book of the New Tang Dynasty was translated into Yu Hongzhi by Shiliang Zhou, while the Book of the Old Tang Dynasty was mistaken for Yu Zhihong. Volume 245 of Zijian is the fish record in November, 1999, while volume 246 of Zijian is the fish record in January, 1995, which are confused with each other.