The basic principle of switching power supply EMI filter was described. The circuit principle of passive EMI filter and several topology circuit of active EMI filter were introduced. Active EMI filter with feedforward, active EMI filter without current detection sensors installed and active EMI filter for three-phase inverter motor were researched. The problem that following the frequency rising of switching power supply, the EMI noise pollution was becoming more and more serious than before was solved. These growing pollutions had resulted in widespread application of EMI filter technology and in the meantime higher requirements were raised to EMI filter technology.