In view of the fact that the spectral resolution of the sub (super) synchronous component of the engineering site is sparse,this paper summarizes the inadaptability of the existing algorithm engineering application and proposes an engineering method of FFT calculation.The effect of spectral aliasing is first eliminated by combining the low-pass filter and the FFT data window length,and then eliminating the interference of fundamental frequency spectrum leakage to harmonic calculation through the decreasing characteristic of FFT calculation results.Finally,using the FFT correction algorithm to eliminate the influence of the fence effect to accurately calculate the frequency and amplitude of sub (super) synchronous oscillation harmonics.Meanwhile,according to the FFT spectrum amplitude of the number of extreme occurrences to distinguish between the sudden changes in the operating conditions of the amplitude,as a basis for the development of monitoring devices to meet the grid sub (super) synchronous oscillation of the precise monitoring of functional requirements.