The basic principle of single-phase active power filter（APF） is introduced and a new single-phase harmonic detecting method is presented.Based on the instantaneous reactive theory

slide average filter is used to pick up DC component by constructing two-phase system in single-phase system.This method can use historical data effectively and decrease the calculation.It has good real-time performance and is easy of digital control.The triangular wave comparison mode is used in output control to decrease output harmonic effectively.The simulation results with Matlab show that the proposed detecting method used by singlephase active power filter can get good compensation performance.