The spectral leakage from asynchronous sampling is the main source of the measurement error for the phase difference correction method in the power harmonic analysis undergoing constant sampling. When making continuous measurement of the power signals in the wide frequency fluctuation range

the measurement of phase difference correction method may fail. An improved method based on adaptive sampling is presented. The real-time fundamental frequency is predicted according to the fundamental frequency of the previous measurement and the rate of change of frequency calculated in the previous measurement. The sampling rate is adjusted according to the predicted fundamental frequency in time to make the sampling rate match the real-time fundamental frequency. The variable-frequency power signals are simulated under the conditions of adaptive sampling and constant sampling

respectively. The results show that the accuracy of amplitudes is improved by one order of magnitude

that of phases 3 to 14 times and the time length of the window is 40% of that prescribed by IEC standard. This method reduces the spectral leakage from dynamic fundamental frequency offset and makes the phase difference correction method accurate and quick in the continuous harmonic measurement of the power systems in the wide frequency fluctuation range.