Abstract: The paper proposed a detection method of power harmonic based on wavelet packet transform. The method uses wavelet packet transform to decompose current signal, namely the current signal is departed into low-frequency part and high-frequency part, then respectively makes wavelet packet decomposition for the low-frequency part and the high-frequency part and gets fundamental component of the current signal after reconstruction. The fundamental component is subtracted from the original current signal so as to get harmonic component of the current signal. The simulation result showed that the method can detect harmonic component of current signal well and can detect harmonic of certain frequency.