The traditional harmonic detection algorithm is affected by noise, resulting in low detection accuracy and easy distortion at the boundary. In this paper, based on successive variational mode decomposition, a harmonic detection method combining wavelet denoising and characteristic waveform matching extension is proposed. Firstly, the adaptive wavelet threshold function is used to smooth the signal noise and eliminate the interference of bad data to the decomposition results. Secondly, the characteristic waveform matching extension method is used to extend the edge of the signal and then cut it to curb the distortion at the end of the waveform caused by the boundary effect. Finally, the harmonic signal is detected by successive variational mode decomposition, the amplitude-frequency information of steady-state harmonics is extracted, and the start-stop time of transient harmonics is located. Simulation results show that the proposed method can effectively reduce the noise interference and reduce the waveform distortion caused by the boundary effect. In electric arc furnace example signal simulation，the average amplitude error and the average frequency error are 0.545% and 0.146% respectively.