Abstract: In order to bridge the contradictory record of the area of the feudal system in Zhou Li (《周礼》)and Book of Rites (《礼记·王制》), Zheng Xuan put forward the concept of " Shefa” (设法) in the notes. Shefa developed into the system thought of the unification of principle and flexibility in traditional China. In Song Dynasty, it combined with Confucianism in understanding Classics. " Division of its meaning without division of its track” was highlighted. " Spirit of system” derived from it. The research of Shefa is very important to understand the progressive system design such as Lijia (里甲), Baojia (保甲) and militia. It also has political thought and methodological significance.