The most important calligraphy theory in the early Tang Dynasty was Sun's Book Score. This 3700-word magnum opus is profound and rich in content, involving all important aspects of China's calligraphy. Its insights are incisive and original, revealing the essence of calligraphy art and many important laws. It has become a landmark work in the history of ancient calligraphy theory in China, marking the development of China's calligraphy into a brand-new brilliant stage. Sun Shi's main theoretical contribution lies in his consistent and scientific exposition of the "expression" essence of China's calligraphy in Shu Pu.