IEC standard adopts 10 cycles’ group or sub-group algorithms to measure harmonics and inter-harmonics, but it cannot accurately measure the frequency, amplitude and phase parameters of each component, and exists large errors with non-synchronous sampling. Through transforming the twiddle factor of signal in frequency domain with IEC standard and non-synchronous sampling, according to frequency point energy distribution, this paper proposes the method of frequency point energy interpolation based on twiddle factor transform, which eliminates the side lobe interference of calculation component came from non-attention components and achieves harmonic and inter-harmonics accurate identification and precise measurement. Simulation results verify that the proposed algorithm is effective and has eminent anti-noise performance, meeting the requirements of IEC standard and engineering measurement.