Abstract: The suppression of narrow-band interference is one of the key techniques of on-line monitoring of partial discharge.In this paper,the different characteristics of fast Fourier transform（FFT） power spectrum between simulation PD signals and PD signals mixed with the narrow-band interference were analysed.The notion of partial peaks was brought in and partial energy ratio was defined.Thus,a method of suppressing narrow-band interference using partial energy ratio pretreatment was proposed,and the steps are also designed.Besides,the magnitude of partial energy ratio is fuzzified and the adaptive fuzzy C-mean threshold algorithm was proposed to extract the threshold of partial energy ratio.The denoising simulation shows that the proposed method could pick narrow-band interference signal from real PD signal.The method of this paper was used in wavelet packet power spectrum to denoise narrow-band interference,comparison with that of wavelet packet denoising shows that the FFT is superior in suppressing narrow-band interference.