Abstract: The time scale determines the results of unit load allocation of hydropower stations. This study first elaborates on the time and space scale effect to disclose the impact of the time scale effect on unit load allocation. It proposes quantitative indicators of the time scale effect, including water consumption, crossing vibration section times, unit startup and shutdown times, and computing efficiency. The ZM hydropower station was selected as a case study, and we performed a time scale effect analysis on a typical day, a typical day with random load fluctuation, and load allocation under different levels. Results indicate that (1) the finer time scale can better characterize the fluctuation of load and reservoir operation, which induces larger water consumption, times of crossing vibration section, and unit startup and shutdown. In contrast, the longer time scale makes it easy to overlook the unit action of crossing vibration section and unit startup and shutdown, which results in the latent risk to the hydropower station. (2) The time scale phenomenon doesn't change with the load fluctuation. It is worth noting that the impact of load fluctuation is nearly equal to the time scale. (3) The time scale effect is relatively smaller under lower or higher load levels due to the fewer action times of unit operation than under medium load levels. Results can provide a reference for the time scale selection of unit load allocation of hydropower stations in both power scheduling formulation and dimension reduction algorithm development.