One of the sources: Zhang's ancestors were the inventors of bows and arrows. According to legend, Shao Hao's son is called "Swing". He saw the curved stars in the sky at night. Because the arc stars are arranged in a bow shape in the sky, "Dang" was inspired by it, invented the bow and arrow, and took the post of bow leader, so he took "Zhang" as his surname. The original meaning of Zhang is to bow.
The second source: During the Spring and Autumn Period, there was a doctor in the State of Jin, and his descendants took the word as their surname, so their surname was Zhang.
The third source: Zhang Youji changed his surname to Zhang, which is famous. Sean, formerly known as Ji Niang, was a Korean aristocrat in the Warring States Period. After the assassination of Qin Shihuang failed, he changed his surname to Zhang, fought for the world from Liu Bang, and finally overthrew the Qin Dynasty, wiped out Chu overlord Xiang Yu, and helped Liu Bang establish the Han Dynasty empire.
The fourth source: it also comes from changing the surname. During the Three Kingdoms period, a veteran of Cao Cao was named Zhang Liao, and Zhang Liao's surname was Nie instead of Zhang, so the surname of Nie in this branch was changed to Zhang.