Abstract: Focusing on the problem of spectrum leakage in Fourier harmonic analysis caused by rapid broadband changes in frequency and harmonics in isolated network under unstable condition, an analysis method for frequency and transient harmonics is proposed based on adaptive periodic sliding filter with variable bandwidth. Firstly, the high-frequency periodic interference of non-stationary signal is eliminated by the preprocessing system based on the principle of periodic sliding filter, and the DC component is separated. Then the preprocessed sequence is locked in real time by zero crossing method, and the bandwidth of the periodic sliding filter is adjusted adaptively. Finally, according to the real-time frequency of the calculated non-stationary signal, the non-stationary signal is divided into harmonics by the discrete Fourier transform (DFT) algorithm, and the transient information of each harmonic component is obtained accurately. Simulation results and field measurement data show that the method has high harmonic measurement accuracy under a variety of non-stationary states, and can effectively measure the frequency and transient harmonics of isolated network under the unstable state.