Abstract: Accurately quantifying the multi-period regulation capacity of cascade hydropower stations under the coupling effect of complex multi-dimensional operating conditions in hydroelectric power systems is crucial for the efficient accomodation of new energy and power grid supply security in southwest China.To address this issue, this paper proposes a quantitative characterization method for the multi-scale regulation capacity of hydropower under multi-dimensional operating conditions.At the short-term and medium-term scales, a quantitative characterization framework for hydropower regulation capacity is established from three dimensions (power, energy, and time), incorporating indicators such as forced output, peak capacity, upward/downward regulation reserve capacity, peak energy, and peak duration.At the long-term scale, a calculation model for cascade hydropower energy storage is developed.Taking the Yalong River cascade hydropower stations as a case study, the impacts of multi-dimensional operating conditions including water level, generating flow, and hydraulic-to-electric energy conversion efficiency on the multi-scale regulation capacity of hydropower were investigated.The results show that at the short-term scale, the lowest water level state that enables the maximum power output corresponds to the strongest peak capacity, and the proposed method effectively mitigates the risk of unreliable implementation of upward regulation reserve plans required for hydropower.At the medium-term scale, further drawdown of the month-end water level for hydropower stations is conducive to meeting elevated peak power demand and prolonged peak load hours.At the long-term scale, energy storage is positively correlated with water level.Due to the leverage effect of "repeated power generation by a single drop of water", the water level of the upstream leading station has the most significant impact on the energy storage capacity of the cascade system.