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Nowadays, the Fast Fourier transform (FFT) has been widely adopted for harmonic analysis in power system. However, a direct application of FFT-based algorithms may bring about inaccurate results because of spectral leakage and picket-fence effects. In this paper, a high accuracy method based on the all-phase and interpolated FFT is presented for harmonic analysis. Firstly, the input signal is preprocessed to mitigate spectral leakage through all-phase. Secondly, the interpolation algorithm is adopted to eliminate the picket-fence effects. As a result, the analysis accuracy of the harmonic parameters has been greatly improved and the phase estimation is not biased. The formulas of amplitude, frequency and phase are also obtained based on all-phase and interpolated FFT. The simulation results reflect that this method can better meet the requirements of harmonic detection in power system.