Abstract: In order to reduce the influence of spectrum leakage caused by non-synchronous sampling in FFT and improve the accuracy of harmonic detection between harmonics, a combinational analysis method based on multi-spectral interpolation algorithm and improved complex modulation method is proposed in this paper. Firstly, the long-range leakage of fundamental and harmonics is estimated and removed, and then the spectral lines around the target frequency are analyzed. The information of these spectral lines is fully used to derive the multispectral interpolation algorithm, and polynomial interpolation The fitting method gets its modified formula. For the signals with very close frequency, the detailed analysis is carried out on the basis of the complex modulation refinement method based on the complex analysis band-pass filter. Simulation and experimental results verify the effectiveness and accuracy of the proposed method.