The large reservoirs are prone to cause some ecological and environmental problems due to water temperature stratification. However, the water depth in the reservoirs is very difficult to measure and little on-site observed data concerning the stratified reservoir flow are available, which is thus difficult to disclose its mechanism thoroughly. An experimental testing system is developed to simulate the formation and development of the water temperature stratification in late spring and early summer. And analysis of the influences of the discharge and outlet position on the vertical and longitudinal distributions of the water temperature has been carried out. The research results show that: ① due to the characteristics of the stratified flow and the differences between hydrodynamic conditions, the vertical structures of the water temperature show various forms in the evolution of the temperature stratification; ② the vertical temperature difference depends on the lower or upper locations, reservoir inflow and outlet elevation. Usually in the vertical direction, the upstream temperature difference is larger than that of the downstream. The vertical temperature difference increases with the increase in the discharge of the intake and the outlet elevation; ③ the longitudinal distribution of the water temperature is closely related to the water depth, discharge and outlet elevation. At the surface and mid-depth in reservoir water, the upstream temperature is higher than that of the downstream. At the bottom water stratification, the downstream temperature is higher than that of the upstream in a certain period when the outlet is located over the surface and the discharge is larger; ④ the water temperature of the oulet is affected by the discharge of the intake and the outlet elevation. A lower outlet location may result in lower outflow temperature, especially in the beginning period.