In the harmonic analysis of power system, the accuracy is badly influenced by spectral leakage and fence effect during FFT transform. The current analysis method of windowed FFT interpolation can suppress the spectral leakage to a certain degree, but it has a complex interpolation formula, meanwhile, different frequency components have mutual interference damaging the effect of harmonic analysis. This paper presents a bidirection Newton interpolation method which can synchronize the sampling sequence. In the case of asynchronous sampling, positive Newton interpolation is used to calculate the primitive period of periodic signal, and then setting new interpolation point according to the calculation of the period, the signal amplitude can be calculated using the reverse Newton interpolation. The adjusted sequence is approximately like that acquired under synchronous sampling condition, so that can effectively reduce the spectral leakage during Fast Fourier transform. The simulation results show that, the method can solve the leakage of spectrum and the harmonic detection has a high precision.