Abstract: An effective time-of-use (TOU) electricity pricing mechanism is essential to stimulate customers to provide load flexibility by price signals, and can directly determine users' electricity cost. TOU electricity price has been adjusted in many provinces in China recently. This paper introduces and analyzes the new round of adjustment. Based on the difference-in-differences (DiD) method, the TOU effect on customers' electricity cost is calculated. Furthermore, enabled by a multiple regression model, the effect of TOU mechanism on electricity cost is revealed by fitting the decoupled coefficient obtained by DiD. Based on the real electricity cost of TOU and non-TOU customers of specific provinces, types, and voltage levels in July 2022 and July 2023, the TOU effects and general trends are obtained. Then, the accuracy of the regression model is verified. Finally, the TOU effect on the cost per kilowatt hour of commercial customers under different price ratios is analyzed. It is found that there is an optimal critical peak and peak ratio to minimize the electricity cost for commercial customers.