Grounded on the theory of power equilibrium, a novel single phase series active power filter (APF) is proposed . By analyzing the power equilibrium character of series APF, the control algorithm and equation are deduced. In the proposed control strategy, instead of the fundamental current detection and complex computation, only the capacitance voltage and the source current on d.c. side are needed. The clear control loop has been realized with simple circuit and the series proposed APF can compensate both reactive and harmonic current simultaneously. The simulation results show the excellent compensation effect and confirm the validity of theoretical analysis.