 He was the sixth son of father Qian Chuanguan (King Wenmu), who was then was still serving under his father (Qian Hongzun's grandfather), Wuyue's first king Qian Liu (King Wusu), as the acting military governor ("Jiedushi") of Wuyue's two main circuits, Zhenhai (鎮海, headquartered at Wuyue's capital Hang Prefecture (杭州, in modern Hangzhou, Zhejiang)) and Zhendong (鎮東, headquartered in modern Shaoxing, Zhejiang).His mother was Qian Chuanguan's concubine Lady Xu Xinyue.While he was ranked as Qian Chuanguan's sixth son, traditional histories heavily implied that he was the second-born in terms of Qian Chuanguan's biological sons, as they emphasized that Qian Chuanguan and his wife Lady Ma were sonless and that, therefore, Qian Chuanguan was sonless into his 30s because Qian Liu had forbidden officials from taking concubines, until Lady Ma personally pleaded for an exemption for Qian Chuanguan due to that reason — and listing Qian Chuanguan's fifth son Qian Hongzun and Qian Hongzuo among the biological sons born from Qian Chuanguan's concubines, while omitting their older brothers Qian Hongzhuan (), Qian Hongxuan (), Qian Hongyou (), and Qian Hong'an (), although only Qian Hongyou was explicitly stated to be an adoptive son.