Active power filter (APF) is the key equipment to realize power quality management in power system. Harmonic detection is one of the key technologies of APF. The detection accuracy and response speed directly determine the overall performance of APF. In this paper, the traditional ip-iq harmonic current detection algorithm based on instantaneous reactive power theory is introduced; and the detection accuracy is affected by the phase error of the phase locked loop output and the response speed of the harmonic detection system is limited by the existence of low-pass filter. An improved harmonic detection algorithm is proposed by frequency-locked loop technology. The algorithm can generate unit sine and cosine signals with the same frequency of grid voltage, and moving average filter (MAF) is used instead of low-pass filter to improve its response speed and harmonic detection accuracy. The algorithm is analyzed theoretically and simulated by MATLAB. The theoretical analysis and simulation results prove the effectiveness of this method.