At present, the One-Cycle Control of Active Power Filter has two serious problems. One is that this method can just compensate the harmonic and reactive power at the same time, which can not be eliminated separately. The other is that the tracking current will be affected and have huge error when the mains voltage appears distortion. So, through the hard work, in this paper, we adopt the method to cover these shortages, which combines an instantaneous detection method with the OCC technology together. The instantaneous detection method is based on trigonometric function, which can real-time monitor the harmonic and reactive components of the load currents and can eliminate the affection of distorted mains voltage. The One-Cycle Control is the current tracking control of APF. Through the theoretical analysis, and the simulation by the PSCAD / EMTDC, the results confirm that this approach can achieve the compensation for the harmonic currents and reactive currents separately, and the effect is good, which has certain practicability. Keywords-Active Power Filter; One-Cycle Control; triangle instantaneous detection method