It is difficult to achieve the synchronized sampling and integral period truncation analysis of tradi-tional harmonic，harmonic parameters calculated to bring inevitable errors，windowed interpolation algorithm can beimproved by non-synchronous sampling the spectrum leakage and fence effect. On the basis of the FFT transformtheory，we adopt the dual spectrum lines windowed interpolation FFT algorithm to estimate the parameters of inter-harmonics，and give out an interpolation modified formula of harmonic frequency，phase and amplitude of Nuttallwindow. The simulation of harmonic signal reveals that this algorithm can achieve accurate detection of Inter-har-monks benefited from accurate analysis of harmonic，and has higher precision than the other compared windowfunctions.