This paper addresses the principle and control topology of active power filter (APF). The control strategy of direct power control based on DC side is introduced, and the control output is implemented by using the principle of hysteresis control. The active power filter can effectively control the power system harmonics, improve the quality of electricity and purify power grid. The experimental results are provided to illustrate that the direct power control based on DC side has the advantages of simple structure, stable DC output voltage and fast dynamic response.