Aiming at the suppression of spectral leakage caused by non-synchronized sampling or non-integral period truncationin FFT-based harmonics detection method, this paper proposed a new harmonics detection method based on spectrum interpolation of apFFT (all-phase FFT). Three discrete spectral lines near the real frequency component was used to correct the frequency and amplitude of power harmonic. By this method, it is not necessary to perform the harmonic phase correction because the peak spectral line phase of discrete apFFT spectrum was just in correspondence with the actual phase of harmonic component. On the basis of the square attenuation characteristics of the main lines in the apFFT spectrum and the fast sidelobe attenuation characteristics of Nuttall window, the interference between adjacent harmonic components in FFT spectral line was effectively suppressed,resulting in the improvement of harmonics detection accuracy. 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.