In consideration of the non-sinusoidal distortion that often exists in practical power system signals， a method for time-varying harmonic detection using Harr wavelet is put forth in this paper. The method employs Harr wavelet bases to approximate amplitudes and phases of the time-varying harmonics instead of decomposing them into different frequency banks. The algorithm of the time-varying harmonic detection for power system is derived as well as the flowchart of the process is provided in detail. Finally， the feasibility of the algorithm is verified by simulation and experimental comparison.