Abstract: Under the coupled influence of complex multidimensional operating conditions in hydropower systems, accurately quantifying the multi-cycle regulation capacity of cascade hydropower is of significant importance for the efficient integration of renewable energy and reliable power supply in Southwest China. Accordingly, this study proposes a quantitative characterization method for multi-timescale hydropower regulation capacity under multidimensional operating conditions. At short-term and medium-term timescales, a three-dimensional quantitative characterization method encompassing power, energy, and duration has been established, including must-run generation capacity, peak shaving capability, up/down regulation reserve capacity, peak shaving energy potential, and peak shaving duration. At the long-term scale, an energy storage calculation model for cascade hydropower has been developed. Taking the Yalong River cascade hydropower station as an application example, this study investigates the impacts of multidimensional operating conditions (water level, discharge flow, and hydro-mechanical efficiency) on multi-timescale regulation capacity. The results demonstrate that: at the short term, the minimum water level that can reach the maximum output corresponds to the strongest peak capacity, and the method described in this paper effectively addresses the risk of unreliable implementation of hydroelectric power reserve plans.; at the medium-term scale, lowering the end-of-month reservoir water level better satisfies the requirements for higher peak shaving energy and longer peak shaving duration; at the long-term scale, energy storage shows a positive correlation with water level, and due to the "lever effect" of one-drop-multiple-generation, the water level of the upstream leading reservoir exerts the greatest influence on the total energy storage of the cascade system. This research provides valuable references for practical power grid dispatch applications.