In a sense

the spectrum leakage is inevitable in asynchronous sampling.A FFT-based iterative algorithm is presented in this paper to solve the problem of spectrum leakage on analyzing periodical signal.The leakage will be reduced sufficiently

and so much as to be eliminated when put to use.The algorithm can be easily applied to determinate accurately the amplitude

the phase of the harmonic component of a distorted signal

and doesn't need more sampling-data.It approaches the accurate amplitude and the phase only by the analysis of the distorted amplitude and phase and has its better theoretical and practical importance.