In order to guarantee the safe operation of power system, it must be detect and analysis the harmonic accurately. Usually the harmonic signal consists of fundamental wave, every harmonic and noise. The existence of much noise will seriously affect the detection precision of the harmonics in power system. In view of the traditional soft threshold and hard threshold de-noising methods have shortcomings, it puts forward a kind of based on sym8 improved wavelet threshold de-noising method. First, the signal decompose on 5 layer by sym8 wavelet, then the detail coefficients of first and second layer use this method to de-noising optimization. Finally, the harmonic detected by the optimized coefficient of each layer reconstruction. In the Matlab platform by comparing the de-noising effect of the soft threshold, hard threshold and improved threshold, it shows that the de-noising effect of improved threshold function is better. With this method improve the accuracy of detecting harmonic obviously after de-noising.