In synchronous sampling conditions, the spectrum leakage of the inter-harmonic and inter-harmonic between is the main reason for the error. In this paper, a new calculation method was proposed to eliminate the interference between harmonic and inter-harmonic.It obtains accurate quasi harmonic signal, by meaning of exponential fitting to estimate the leakage value of the inter-harmonic at the adjacent to the line of the harmonic spectrum. The quasi inter-harmonic signal is obtained by separating the quasi harmonic signal through the time-domain sampling method. The inter-harmonics of frequency relatively nearby divide the frequency bands by the null method,and for each frequency band, the method of adding window interpolation is used to obtain the more accurate harmonic parameters. The simulation error results have verified the effectiveness of the proposed method.