Wang Wujun
It is not known when Wang Chengzong was born, although it was known that he was born before his brother Wang Chengyuan, who was born in 801. He was the oldest son of Wang Shizhen, who was then deputy military governor of Chengde under Wang Chengzong's grandfather Wang Wujun. After Wang Shizhen succeeded Wang Wujun as military governor in 801, Wang Chengzong eventually became the acting governor of Chengde's capital Heng Prefecture (恆州, in modern Shijiazhuang, Hebei) as well as deputy military governor of the circuit — effectively, Wang Shizhen's designated successor.