An approach for power harmonic analysis based on 8-term cosine window interpolation FFT algorithm can further improve the precision of harmonics measurement, and the paper provides a new interpolation FFT algorithm based on 8-term cosine window. It firstly analyzes the spectrum characteristics of 5 to 8-term cosine window and then deduces the interpolation algorithm on 8-term cosine window in detail, the cubic spline function is adopted to calculate the frequency and the harmonic amplitude modification coefficient, and calculating burden is reduced. The results show that, compared to other cosine window interpolation FFT algorithm, the interpolation FFT algorithm based on 8-term cosine window has higher accuracy, and the results verify the effectiveness and practicality of the proposed algorithm.