To study the impact of cascade hydropower development on water temperature in the Lancang River Basin, a 1D-3D coupled water temperature model was constructed, and the spatiotemporal variation of water temperature in the cascade reservoir system from Gongguo Bridge to the international boundary section of the Lancang River was systematically simulated. Results indicate that water temperature changes exhibit a continuous and cyclical process:reservoir heat storage in deep-large reservoirs-low temperature discharge upstream of dams-natural warming, demonstrating significant cumulative effects. The deep cascade reservoirs slow down the flow velocity of incoming water, and the thermal storage effect causes the annual average surface water temperature of the upstream Xiaowan Reservoir to increase by 1.5℃ compared to the natural state. Due to the stratification of water temperature and the discharge of low-temperature water in deep-large reservoirs, the annual average temperature difference between the discharge water temperature and the natural temperature in Xiaowan Reservoir is -2.4 ℃. The temperature difference in the fluctuating catchment area rises to -1.9℃ with the temperature and radiation, posing a year-round low-temperature stress on the fish habitat under the dam of Xiaowan Reservoir. The downstream Nuozhadu Reservoir further strengthened this process, with an average annual increase in water temperature in front of the dam of 1.7℃, and the average annual discharge water temperature difference narrowing to -1.6℃ compared to the natural temperature. Under the combined effect of latitude and meteorological gradient, the thermal recovery capacity of the fluctuating catchment area is better than that of Xiaowan Reservoir, resulting in an average annual water temperature of 0.3℃ higher than the natural temperature in the section from the dam to the exit of Nuozhadu Reservoir. From August to January of the following year, it poses a high temperature stress on fish habitats. The cumulative effect of cascade hydropower development on water temperature has led to a shift from a single low temperature stress to a dual stress of low and high temperatures. Keywords: cascade hydropower development; water temperature stratification; water temperature residence time; flattening effect; the Lancang River 〖FL