Abstract: Water temperature is an important factor in water environment assessment, and impoundment of a dam usually poses a significant impact on the water temperature in the reservoir and its outflows. This study develops a two-dimensional CE-QUAL-W2 temperature model and analyzes the characteristics of water temperature based on the field data observed at the Xiluodu Reservoir, focusing on evaluating the variations in its outflow temperature in terms of baseline deviation index, phase shift index, and extreme value amplitude index. Results show that the vertical thermal profiles measured in-situ are different from previous predictions. The surface epilimnion varies seasonally from 50 m to 100 m in water depth and from 15℃ to 23℃ in water temperature, sensitive to the water temperature and flow rate of reservoir inflow, and in the surface layer no obvious temperature gradient is observed. Stratification in water temperature varies seasonally from March to October. Typically, thermocline moves downward from the initial elevation of 520m to 430m and its thickness correspondingly decreases from 20 m to 10 m. This downward movement is faster when water is discharged through the deep tunnel spillway in flood season, while gradual when the water temperature of surface layer is maintained at 14℃-15℃. After the impoundment of Xiluodu, the temporal process of discharged temperature delays up to 36 days, and its peak value is 2℃ higher and lowest value 1℃ lower than that of the natural river. This study concludes that the prominent impact of the Xiluodu reservoir on the temperature dynamics of its outflows is temperature lagging and temperature homogenization.