Wuxu was born to a Di (翟) slave of Jianzi of Zhao (趙簡子). Despite his low status, his abilities gained the recognition of Gubu Ziqing (姑布子卿), an advisor of the house of Zhao, who recommended him to Jianzi. Gradually, Wuxu became Jianzi's favourite son. Once, Jianzi informed all his sons about a hidden treasure, a precious fu (符, a military seal) on Mount Chang (常山, modern Mount Heng), and promised a reward for whoever recovered it first. While all others returned empty-handed, only Wuxu grasped his father's true meaning - the strategic terrains there served as a perfect basis for a military campaign against the state of Dai, whose lands were considered the true prize. This event led Jianzi to name Wuxu as his successor, instead of the former heir, Bolu (伯魯).