Aiming at the problem of significant deviation between the calculation results of abandoned water risk in existing hydropower stations and the actual situation, based on the principle of water balance, a cascade reservoir abandoned water risk quantification method coupling runoff and load dual uncertainties was proposed by identifying abandoned water risk factors. The concept of critical abandonment probability was introduced, and a method for determining the critical abandonment probability of cascade reservoirs based on the joint minimum of warning false alarm rate and missed alarm rate, as well as a method for controlling the water level drop of cascade reservoirs at the end of the water supply period based on the critical abandonment probability, were proposed. On this basis, a mid- and long-term multi-objective optimal operation model considering the abandoned water risk in cascade reservoirs was constructed, and a case study was conducted on the Wujiang cascade reservoir group. The results show that there is a significant monotonic increasing relationship between the abandoned water probability and the abandoned water flow rate in the Wujiang cascade reservoir group. The two-stage scheme of coupling operation period and residual period has increased the power generation efficiency by about 0.51% compared to the conventional optimized scheduling scheme, and the ecological efficiency has increased by about 1.06 times.