Abstract: The inter-harmonics in power grid may cause negative influence on power quality and power supply reliability,so the accurate detecting of inter-harmonics is important for the stable operation of the power system.According to the inter-harmonics characteristics,based on the FFT algorithm,interpolation FFT algorithm was proposed to detect inter-harmonics.Through the analysis of the characteristics of different window functions,Hanning window is selected as the analysis window,and the window-length and the sampling period are determined,which can detect harmonics and inter-harmonics accurately.Besides,FFT algorithm and interpolation FFT algorithm are simulated by MATLAB.By contrasting and analyzing the estimated results of frequency and amplitude,it can be seen that the windows and interpolated FFT algorithm can detect harmonics and inter-harmonics of a given signal much more accurately.