Non synchronous sampling is the cause of spectral leakage and fence effect, and the spectral leakage and fence effect is the main factor influencing the DFT transform harmonic measurement precision. So it is very important for the accuracy of sampling method. The problem of the error of the traditional method for sampling, this paper proposes an improved double speed synchronous sampling the method, periodic error suppression method of periodic signal generated in the sampling interval using the traditional compensation method, the residual error compensation is analyzed, comparing the residual size error of timer resolution Td, so as to make further compensation, make the RMS spectrum, thus more effectively reduces the error at last, the simulation results verify the validity of the proposed method.