Abstract: The impact of resource endowment on resource exploitation efficiency and its influencing path is always a key scientific problem in the process of resource exploitation. In this paper, we established an efficiency evaluation method for cascade hydropower development based on data envelopment analysis. The efficiency of cascade hydropower development was taken as the explained variable. Water resource endowment was taken as the core explanatory variable, which included the runoff amount and runoff spatial pattern. Hydropower development mode was taken as the mediating variable, which included intensive development mode and diverse development mode. We chose the main hydropower development watershed as the case areas, including 331 large and median-sized hydropower plants and formed the panel data of 22 watersheds. The mediating effect model was used to verify the influencing path of water resource endowment on efficiency of cascade hydropower development. The results showed that: (1) Taking the expected and unexpected outputs of hydropower development into consideration in the efficiency model, the efficiency of cascade hydropower development varied greatly among watersheds, most watersheds were in low or relative low efficiency, and the average efficiency was only 0.258 during 1996-2020. (2) Water resource endowment had a significant impact on the efficiency of cascade hydropower development. The runoff amount posed a positive impact on the efficiency of cascade hydropower development, and the intensive development mode played a mediating effect towards this positive impact. Meanwhile, the runoff spatial pattern posed a negative impact on the efficiency of cascade hydropower development, and the intensive development mode and diverse development mode all played a mediating effect towards this negative impact. (3) The runoff amount was beneficial to the intensive development of hydropower, but the runoff spatial pattern was indicated as disadvantageous to the cascade development of hydropower. The intensive development mode and the diverse development mode were all beneficial to improve the efficiency of cascade hydropower development under the suitable water resource endowment condition. These conclusions provided theoretical basis for optimizing the spatial pattern of watershed cascade hydropower development.