In the region of the harmonic parameter analysis, we often use the Fast Fourier Transform (FFT) to get the character of the signal, but due to the limit conditions of the FFT, the analysis results will appear some error effects, these error effects are: Aliasing effect¡BLeakage effect and Picket-fence effect. When the error effects appear, we can not get the harmonic parameter accurate. For the purpose of getting accuracy harmonic parameters, In this paper proposes, we will introduce the analysis method of FFT and it's limit conditions, then will based on FFT to develop an accuracy formula to find the harmonic parameter fast and accurately. This method will be divided into three parts. First, we will discuss the effect of harmonic to spectrum. Second, we will discuss the parameter calculation with no interfere. Third, we will rid interfere of the harmonic for getting accuracy harmonic parameter. Finally, we will analysis the actual signal to prove the ability of this method, and raise the future research direction for the successor.