Numerical simulation of flow field,temperature field,concentration field and ecological process is a powerful tool for operation,management and environmental protection of reservoir based on EFDC. The hydrodynamic model is utilized to simulate the temperature field of Mopanshan reservoir under unsteady flow condition and all the numerical simulations are carried out. The results show that the reservoir is under the condition of thermal stratification in the whole year. In April the temperature difference in vertical is small. In June and July the water close to the surface forms a thermocline,which is a barrier of mass transfer in vertical. In September and October the mixing of water in different temperature layers is strengthened and it has a large effect on water quality. The study provides a reference for locating the water intake with seasons. It is suggested that water plants should enhance the preprocess in high water period in September and October.