Abstract: For the problem that traditional Fourier transform method caused big errors when analyzing power signals of power network under unstable running condition, a harmonic detection method for power network was proposed based on wavelet transform Mallat algorithm. Firstly, power signals are separated into different sub frequency bands according to resolutions. Then each sub frequency band is reconstructed to get fundamental wave of original signal. Finally, the reconstructed signal of fundamental wave is subtracted from the sampled original signal so as to get harmonic signal. The Matlab simulation results show that the method can separate harmonic signal from fundamental wave of power signal with high precision of harmonic detection.