To assess the impacts of the Danjiangkou Dam heightening on water temperature in the main stream of the Hanjiang River, the measured water temperature data of the Baihe, Huangjiagang and Xiangyang stations in the mainstream of the Hanjiang River was analyzedand the variation degree of water temperature in the lower reaches of the reservoir was quantitatively evaluated based on the improved evaluation indicator values for river water temperature variation. The results show that the Danjiangkou Reservoir heightening has significantly changed the hydrological regime in the middle of the Hanjiang River. The extreme variation value of water temperature under the dam shows a downward trend, and the degree of homogenization increases year by year. The date of the maximum and minimum water temperature at the Huangjiagang station has been postponed, indicating the phenomenon of cold and warm stagnation. The change degree of water temperature regime is closely related to the distance. The Huangjiagang station is closer to the dam, and its water temperature regime changes obviously, and the water temperature concentration period lags about 23 days compared with that before the dam heightening. The Xiangyang station far away from the dam has little water temperature variation regime.