Near the end of the reign of Emperor Yang of Sui, he served as a low-level official in the examination bureau of the government (門下省, Menxia Sheng), and he was honored by both Su Wei and Pei Ju, the head and the deputy head of the examination bureau.
Dai was slightly promoted in Yang Tong's administration, but remained relatively low in his position.
Rather, he made Dai the secretary general of Zheng Prefecture (鄭州, roughly modern Zhengzhou, Henan) to assist his nephew Wang Xingben (王行本) in defending Hulao Pass.
He continued to serve in that role after Wang seized the throne later in 619, ending Sui and establishing a new state of Zheng as its emperor.
At that time, there was an incident where Zhangsun Wuji, Emperor Taizong's brother-in-law and the minister of civil service affairs, forgot to take off his sword when entering the palace, a major violation of regulations.
Emperor Taizong sent the suggestion back to the high level officials for rediscussion, and Feng again proposed the same punishments—execution for the guard and a fine for Zhangsun.
By repudiating your action and putting them in the hands of the law, you are showing that you can hold back your anger and respect the state's credibility.
It was said that Dai often spoke against Emperor Taizong's wishes in this way, and that as a result, people were not falsely convicted or punished while he was serving.
Later in 627, Dai was made a deputy head of the important executive bureau of government (尚書省, Shangshu Sheng).
Dai proposed that Emperor Taizong follow the example of Emperor Wen of Sui and establish large food storages to prepare for famines, and that both the nobles and the regular populace be made to submit a regular tax in either wheat or rice so that the food storages can be filled.
Around that time, the positions of the heads of the executive bureau happened to be both vacant—as Xiao Yu had just been removed from office, and Feng had just died.
In 629, Dai became the minister of census, as well as an advisor to Emperor Taizong's son and crown prince Li Chengqian.
Later in the year, he was no longer the minister of civil service affairs, but he kept his other posts and continued to be designated de facto chancellor.