In order to enhance the safety of the reactor, the passive residual heat removal system (PRHRS) is adopted by both the 10 MW High Temperature Gas-cooled Test Reactor (HTR-10) and the High Temperature Gas-cooled Reactor Pebble-bed Module (HTR-PM). The heat transfer process in the reactor cavity is very important for the whole PRHRS to remove the heat from the reactor, especially once an accident occurs. A three-dimensional numerical model used to simulate the heat transfer process in the reactor cavity of HTR-10 was developed in this study, in which the flow of water in the water cooled pipe was included. The effects of different symmetry boundary conditions on the heat transfer process were studied. Three experimental conditions of HTR-10 were simulated, and the local and global heat transfer processes were analyzed. The results show that the effects of symmetry boundary condition can be neglected when two or more water cooled pipes are included in the model. The temperature of air varies greatly near the reactor pressure vessel (RPV) and the water cooled panel. The maximum temperature occurs at the top of the RPV. With the elevation increasing, the rising rate of water temperature gradually increases. Moreover, radiation become more dominant as the RPV temperature rises.