As the nonlinear load increases, a large number of harmonic will back to fill the grid which may result in the power pollution and measuring inequality. However, the traditional fast Fourier transform （FFT） detection methods are difficult to meet the harmonic detection requirements for accuracy due to the limitation of their own technology. Combined with the characteristics of power harmonic form, this paper analyzed the principle of wavelet transform and Fourier transform, and put forward a kind method with double-spectrum-line and window function based on the wavelet packet combined with FFT. This paper mainly studied the characteristics of different window functions, and added them to get the optimal window function for power harmonic detection by comparison. The simulation results verify that the harmonic measurement method has a good measurement effect in time domain and frequency domain, and the simulation experiment provides a good reference for the detection method of power harmonic of harmonic watt-hour meter and the selection of window function of signal interception.