The power harmonic analysis method DFT-based has been widely used in the harmonic analyzer, but the analysis accuracy is seriously affected for the spectrum leakage due to the complexity of the grid and the error of sampling frequency. The paper presents a new method which used the sampling rate conversion in samples domain to adapt the changes in parameters of grid signals and detection system to overcome the problems effectively. The sampling of given number was started by the zero-crossing trigger, the period of grid signals and the sampling rate change factor were measured by samples(the measuring results also reflected the relative errors); the spectrum leakage problems were overcome after the sampling synchronization by rate conversion. In the end, the effectiveness of the method is verified through simulation and actual data processing.