Abstract: The fluctuation of reservoir water surface has complex flowing boundaries. The ability and effectiveness in processing complex boundary conditions will have a major impact on the sustainable development and utilization of water resources. In order to simulate the large variation of the free surface of the real reservoirs, this research deals with the variation of the free surface of the reservoir by use of the immersed boundary method (IBM), which compares the distribution of the reservoir water temperature, the reservoir drainage temperature and the vertical water temperature distribution in the front of dam, and the results are verified by a simple calculation example. The simulation results show that the method has good feasibility and accuracy, can improve the prediction method for reservoir fluctuating water level, and can be used to study the spatio-temporal variation law of the reservoir water level. And the research results have broad application prospects in dealing with the problems of large variation of the reservoir water surface and can provide a technical support for simulating similar reservoirs in the future.