This paper presents a three-phase four - wire active power filter based on one-cycle control. This method need not sense the three phase load currents or the three phase voltages, need not use any multipliers. It has simplified circuit , and the controller is composed of four integrators with reset along with several flip-flops, comparators and some analog components. The main circuit switching frequency is constant, which is desirable for industrial applications. Analysis and simulation are carried out with the four-arm inverter, the simulation results verify that the APF with one-cycle control can dynamically and effectually compensate the harmonics, reactive current and zero sequence current.