With the development of smart grid, nonlinear load is connected to the grid in large numbers, and the unsteady signal has an impact on the measurement of electric energy. In order to improve the time-frequency resolution of the unsteady signal, this paper improves the Gaussian window scale factor σ according to the characteristics of the unsteady signal and the design principle of the window function. In addition, in order to ensure the accuracy and rationality of the measurement under the unsteady signal, the fundamental energy and the distorted energy should be measured separately. Therefore, this paper proposes an electric energy measurement method based on the improved S-transform, which uses the excellent time-frequency resolution of the improved S-transform to realize the decomposition and reconstruction of the fundamental energy and the distorted energy. Through the simulation of several typical nonlinear load signals, the superiority and effectiveness of the improved S-transform algorithm in electric energy measurement are verified.