It is difficult to perform accurate harmonic analysis with Fast Fourier Transform (FFT) under the unsynchronized sampling conditions. According to the components of the sidelobe characteristic of the Nuttall window and signal discrete spectrum characteristic by FFT, an approach for power harmonic analysis based on Nuttall window and the triple-spectral-line Nuttall window interpolation is proposed. The modified formula is obtained by using polynomial fitting method and Matlab simulation of the power harmonics is made based on it. The simulation results indicate that under the same condition, the amplitude correction errors are less than 2.44×10-5 %, as well as that of the calculation phases no more than 0.004%. It has higher analysis accuracy comparing with single-spectral-line interpolation and double-spectral-line interpolation. Compared to the five points' and the nine points' algorithm, the application rectification formulas is obtained without the need to get the spectral sequence.