武承嗣
It is not known when Wu Chengsi was born. His father Wu Yuanshuang (武元爽) was a half-brother of Wu Zetian—both had, as father, the early Tang Dynasty general Wu Shihuo (武士彠), but Wu Yuanshuang and his brother Wu Yuanqing (武元慶) were born of Wu Shihuo's first wife Lady Xiangli, while Wu Zetian and her two sisters were born of Wu Shihuo's second wife Lady Yang. After Wu Zetian became empress to Emperor Gaozong in 655, despite previous intrafamily unpleasantries—Wu Yuanqing and Wu Yuanshuang, as well as Wu Shihuo's nephews Wu Weiliang (武惟良) and Wu Huaiyun (武懷運) were often disrespectful of Lady Yang previously—Empress Wu's brothers and cousins were often promoted by Emperor Gaozong, with Wu Yuanshuang promoted to "Shaofu Shaojian" (少府少監), the deputy director of the office of palace supplies. However, sometime before 666, Empress Wu, angry that her brothers and cousins did not appreciate the promotions, had them all demoted, with Wu Yuanshuang demoted to be the prefect of Hao Prefecture (濠州, roughly modern Chuzhou, Anhui). Eventually, after further accusations, Wu Yuanshuang was exiled to Zhen Prefecture (振州, roughly modern Sanya, Hainan) and died there in exile. When Wu Yuanshuang was exiled, Wu Chengsi went to Zhen Prefecture with his father.