Aiming to the spectral leakage caused by non-synchronized sampling and non-integral period truncation in FFT-based harmonics detection method, a new harmonics detection method based on spectrum interpolation of apFFT (all-phase FFT) was proposed. Three discrete spectral lines near the real frequency component was used to correct the frequency and amplitude of power harmonic. It is not necessary to perform the harmonic phase correction because the peak spec-tral line phase of discrete apFFT spectrum just corresponds to the actual phase of harmonic component. On the basis of the square attenuation characteristics of the main lines in the apFFT spectrum, and fast sidelobe attenuation character-istics of Nuttall window, the interference between adjacent harmonic components in FFT spectral line can be effec-tively suppressed interpolation algorithm，thus improving the accuracy of harmonics detection. The simulation results verified that the proposed method performed better than the Triple-spectrum-line Interpolation method and all-phase time-shifting phase difference method.