Interharmonics are an important parameter of reflecting the power quality, and there are many measuring algorithms.This paper puts forward a detection method of interharmonics based on the S transform, it is a timefrequency analysis method combined STFT with CWT. Compared with STFT, the window height and width of S transform can be adjusted with frequency. Compared with CWT, phase correction is achieved and the measurement accuracy of Interharmonics is improved. The steady state signal, non steady state signal and different signal to noise ratio (SNR) signal are simulated and analyzed on the MATLAB platform. The frequency and amplitude measurement accuracy of Interharmonics is given in this paper.