When he had become relatively old for a new official, he, on account of his heritage, was made the legal officer at Lu Prefecture (潞州, in modern Changzhi, Shanxi).
Because of his relatively old age, he was not noticed, but was eventually summoned to the Tang capital Chang'an to serve in the ministry of civil service affairs (吏部, Libu), and then to serve as Dali Cheng (大理丞), the secretary general of the supreme court (大理寺, Dali Si).
It was said that because the realm was peaceful at times, each year there would be tens of thousands of people coming to Chang'an seeking official commissions.
Pei was said to be dexterous, knowledgeable, good in memory, detailed, and not harsh, despite the large amount of work he had to carry out, and was said to be the best at the task.
Toward the end of the Tianbao era, the chancellor Yang Guozhong was exceedingly powerful, and he sent most officials who were not his followers away from the capital.
In 761, based on Xiao's recommendation, Pei was made Huangmen Shilang (黃門侍郎) — the deputy head of the examination bureau (門下省, Menxia Sheng) — and given the designation Tong Zhongshu Menxia Pingzhangshi (同中書門下平章事), making him a chancellor de facto.
Pei was soon made minister of civil service affairs (吏部尚書) and You Puye (右僕射), one of the heads of the executive bureau.