Aiming at the problem that the traditional phase difference correction method has a large error in the fundamental wave frequency fluctuation, it may even produce the problem of measurement failure, and an improved algorithm is proposed. The improved algorithm has four improvement measures compared with the traditional phase difference method. The first is to multiply the sidelobe decay rate by multiplying the spectral expression after windowing, which reduces the spectral leakage and the interference between the spectral lines. Second, the correction formula of the traditional phase difference method is further modified by calculating the frequency change rate (ROCOF). The third is to reduce the sampling window length and improve the real-time performance and calculation accuracy of ROCOF when calculating ROCOF. The sub-phase difference method reduces the spectral leakage caused by the non-synchronous sampling caused by the fundamental frequency fluctuation. The text is verified by the improved algorithm in two cases where the fundamental frequency is stable and the wide frequency fluctuation of the fundamental frequency is verified. The simulation results show that the accuracy of the improved electrical parameters is much higher than that of the traditional phase difference method, and the window length satisfies the IEC standard.