Abstract: Temperature is one of the key hydrological parameters affecting the spawning and reproduction of fish. To clarify the impact mechanism of the operation of cascade reservoirs in the lower reaches of the Jinsha River on the arrival times of the critical spawning water temperature and accumulated temperature threshold of fish in the upper Yangtze River, this study targeted three fish species, namely Schizothorax prenanti , Myxocyprinus asiaticus, and Acipenser dabryanus. Based on measured water temperature data, their critical spawning water temperature, accumulated temperature threshold, and suitable spawning water temperature range were determined. The CE-QUAL-W2 model was employed to simulate the spatiotemporal distribution of water temperature under two scenarios: the standalone operation of the Xiangjiaba Reservoir and the combined operation of the Xiangjiaba-Xiluodu Reservoirs in a normal water year. The impact of discharged water temperature changes on the arrival times of the critical spawning water temperature and accumulated temperature threshold of the target fish species was analyzed.The results showed that the changes in discharged water temperature after dam construction significantly delayed the arrival times of the critical water temperature and accumulated temperature threshold for the target fish: under the standalone operation of the Xiangjiaba Reservoir, the arrival time of the critical spawning water temperature was delayed by 21–31 days, and the arrival time of the accumulated temperature threshold was delayed by 5–14 days. Under the combined operation of the two cascade reservoirs (Xiangjiaba-Xiluodu), the delay range of the above indicators expanded significantly: the delay in the arrival time of the critical water temperature increased to 37–49 days, and the delay in the arrival time of the accumulated temperature threshold rose to 13–30 days.This study provides a scientific basis for evaluating the impact of water temperature changes on the spawning and reproduction of fish in the upper Yangtze River and guiding the ecological operation of cascade reservoirs.