In this paper, a simplified equivalent circuit model of active power filter is established. The model can be used to detect positive-sequence reactive current, negative-sequence current, harmonic current, and neutral current of the system. In the proposed model, the reference current can be obtained by the instantaneous power theory rapidly without the separation of the positive and negative sequence components and the extraction of each harmonic current. The quasi-proportional resonance controller is proposed to realize the frequency components of the reference current. To ensure the robustness of control system when frequency fluctuates, the number of sampling in each frequency period is fixed by using the digital phase-locked loop. Case studies on a 0.4 kV/50 A modular active power filter are conducted to verify the accuracy of the proposed model and the effectiveness of the proposed algorithm.