Abstract: A research is proposed for Voltage-mode Buck converter type 3A compensator in this paper.The cross frequency of type 3A compensator is derived in detail,and it gets the cross frequency has nothing to do with load current.When input voltage and output voltage change obviously,to get good phase margin and loop stability,it only to adjust external capacitance Cff of the compensation network.Matlab and Hspice is adopted in loop simulation,and it verifies the theory study of type 3A compensation.