The detection accuracy of time-frequency parameters in non-stationary harmonic analysis is low. To address this issue, a new method based on the adaptive variational mode decomposition (AVMD) and the improved energy operator is proposed. Firstly, AVMD is used to decompose the unsteady harmonic signal, in which the waveform feature matching method is used to extend the unsteady harmonic signal to reduce the influence of boundary effects. The energy difference and correlation coefficient are utilized as the modal decomposition factors in AVMD. Combined with the modal components, an improved sampling energy operator is proposed to quickly extract the instantaneous amplitude and frequency of harmonics, and complete the positioning of its start and end times according to the difference sum signal. In this way, the detection of time-frequency parameters of unsteady harmonics is realized. The simulation and actual measurement results show that the method can effectively complete the accurate detection of non-steady harmonics under the condition of power frequency fluctuation, inter-harmonic and noise interference, and realize the accurate positioning of transient harmonics. The maximum detection errors of non-steady harmonic frequency and amplitude are 0. 094 9% and 0. 931 4% .