The unbalanced voltage conditions deteriorate the performance of the harmonic detection of active power filter (APF). To address this problem, a complex LMS based method is proposed in this paper. A complex Least Mean Squares (LMS) is designed for phase-tracking. DSPOC principle is utilized to adjust the estimate of synchronous frequency. Then, the proposed method is employed to improve the harmonic detection of APF. The effectiveness of the method is confirmed through simulation results.