사조의
Little is known about Zhu Xicai's background, and it is not known when he was born. According to the Tang Dynasty historian Ping Zhimei (), whose "Jimen Jiluan" () is no longer extant but is often cited in other works, he served under Shi Chaoyi, the final emperor of the state of Yan during the An Shi Rebellion against Tang Dynasty rule, and, after Shi Chaoyi had become emperor after assassinating his father Shi Siming in 761, the Yan general remaining at the major city of Fanyang (范陽, in modern Beijing) went through major bloody infighting, at the end of which Li Huaixian emerged the victor. Zhu Xicai was then serving under Li, and he remained at Fanyang to continue to serve under Li after the disturbance. After Li submitted to Emperor Daizong of Tang in 763 and turned against Shi Chaoyi (leading to Shi Chaoyi's committing suicide, ending Yan), Zhu continued to serve under Li.