Abstract: In view of harmonic detection link of four-leg active power filter, the paper proposed a harmonic current detection method based on generalized integrator, and analyzed detection principle of the detection method and influence on detection effect of harmonic current by change of parameter k. The simulation result shows that the detection method can extract fundamental component of harmonic current when frequency of power network is stable; the parameter k influences on rapidity and accuracy of detection, so it should be set reasonably; the detection method has certain adaptability to distortion of power voltage, fluctuation of power frequency and unbalance of three-phase load with simple structure and being easy to realize.