On the basis of serious electromagnetic interference (EMI) produced by power electronic equipment, coupling capacitance existing between transistor and transistor is considered as the main factor disturbing differential-mode (DM) loop. Calculation model of coupling capacitance is established by using FEM (Finite Element Method) and moment method, and computing method is also derived. EMI improvement method for converter system is proposed by controlling the coupling capacitance impedance. Experimental results show that changing the placement distance and position of the transistor can reduce the conducted interference.