There is frequency leakage in situation of non-synchronized sampling and non-interger cycle truncation for fast Fourier transform (FFT) and the high measurement accuracy harmonic parameters can not be detected. To reduce the influence of non-synchronized sampling on FFT, this paper carries out harmonic analysis using the 4-term minimum sidelobe Nuttall window combined with double-spectrum-line interpolation FFT algorithm. The spectral characteristics of 4-term minimum sidelobe Nuttall window are analyzed and an algorithm based on 4-term minimum sidelobe Nuttall window interpolation FFT has been proposed. The applicable formulas are obtained by fitting of polynomials. The simulation results show that the algorithm has a better detection precision than Blackman window and Blackman-Harris window interpolation FFT algorithms in non-synchronized sampling which will control the spectral leakage efficiently.