In recent years, the frequent extreme weather has led to the rise of reservoir water level, and accordingly changed the reservoir water temperature. As for concrete dams, changes in reservoir water temperature and air temperature may generate temperature stress, and such effect and sudden rise of water level will inevitably change the dam force and endanger the dam safety. By means of numerical simulation and theoretical analysis, this paper analyzed the effect of the extreme rainstorm on the reservoir water level and water temperature, and selected a typical project to simulate the changes of temperature field and stress field of concrete dams in an extreme rainstorm under different working conditions. Results showed that: (1) After the storm flood entered the reservoir, the temperature stress changed little due to the change in the reservoir water temperature; (2) During the rainstorm, the compressive and tensile stresses of concrete dams increased with the time, but did not exceed the allowable values of the concrete used for the dam body; therefore, the dam body was safe; (3) By comparing the effects of the reservoir water level rise caused by heavy rain on the dam stress, the dam temperature stress response varied with the water level rising rate: the greater the reservoir water level rising rate was, the greater the maximum dam response stress was. After being affected by the rainstorm, the dam stresses were the same. Therefore, it was necessary to use a dispatching method to control the rise of the water level. This study can provide theoretical basis and reference for the operation and dispatching of reservoirs during extreme rainstorms.