It is not known when Zhang Changzong was born. It is known that his father Zhang Xizang had, at one point, served as the census officer of Tang Dynasty's capital prefecture Yong Prefecture and was a distant nephew of the Zhang Xingcheng. His mother might be Lady Wei or Lady Zang.
As Wu Zetian's lover
In 697, Zhang Changzong was referred to Wu Zetian, then ruling with the title of emperor -- the only woman in the history of China recognized for doing so -- by Wu Zetian's daughter Princess Taiping, to be Wu Zetian's lover. Zhang Changzong further told Wu Zetian that his older brother Zhang Yizhi was even more skillful than he was, and so Wu Zetian took Zhang Yizhi as a lover as well. Both were repeatedly promoted -- Zhang Yizhi as senior consultant at the examination bureau of government -- and both were said to often put makeup on their faces and dressing in a beautiful manner. Their mother or mothers were given titles as grand ladies, and Wu Zetian further ordered the chancellor Li Jiongxiu to serve as the lover for one of them. Even the most powerful officials flattered them, and referred them by endearing nicknames -- Zhang Yizhi as Wulang and Zhang Changzong as Liulang .
By 701, Wu Zetian, then 76 years old, had been largely entrusting the affairs of state to Zhang Yizhi and Zhang Changzong -- a situation that her grandson Li Chongrun disapproved and had discussed with his sister Li Xianhui the Lady Yongtai and Li Xianhui's husband, Wu Zetian's grandnephew Wu Yanji the Prince of Wei. When the Zhangs became aware of the conversation, they reported this to Wu Zetian, who saw this discussion as an implicit criticism of herself, and she ordered Li Chongrun, Li Xianhui, and Wu Yanji to commit suicide.
By 703, the Zhangs were displeased with the chancellor Wei Yuanzhong, as Wei had repeatedly rebuffed the promotion of their brother Zhang Changqi and had publicly humiliated another brother, Zhang Changyi . When Wu Zetian suffered a minor illness, the Zhangs became concerned that if Wu Zetian had died, they would be killed by Wei, and therefore falsely accused Wei and a favorite of Princess Taiping, Gao Jian , of having speculated about Wu Zetian's death. They persuaded the official Zhang Shuo to falsely corroborate the accusations against Wei, although, once Zhang Shuo was brought into Wu Zetian's presence, he not only recanted the accusation against Wei but further accused the Zhangs of suborning perjury from him. As a result, Wei, Gao, and Zhang Shuo were all exiled, but none of the three suffered death. Zhang Yizhi further accused eight people who held a send-off feast for Wei of treason, but was not successful in getting the eight men executed.