Abstract: A model of spent fuel pool in nuclear power plants is developed by using the system code RELAP5/MOD3 and it simulates the hot water behavior of the whole spent fuel pool in the present paper. The nodalization in the model is based on the cycles of spent fuels. Based on the model, normal operation of the spent fuel pool and its cooling system is simulated. The spent fuel pool loss of cooling accident is simulated after that. The effects of radiation heat transfer mode, spray before leakage of radioactive material and the nodalization of hot channel and bypass outside of rack on the temperature response are analyzed. It is shown in the calculation that the model is able to simulate the normal operation of the spent fuel pool and its cooling system and succeeds in establishing the mechanism of natural convection cooling. The fuel uncover time is 17.87 days and it takes 19.14 days for fuel PCT to exceed 1204℃. The 12.6 kg/s spray cooling takes 2.4 hour to reduce the temperature of the fuels from 726.9℃ to 100℃.