Abstract: Gas-liquid flow and heat transfer in a nuclear reactor were investigated by the numerical simulation, on the base of former numerical study of the experimental model in this paper. The Euler two phase flow model, the MUSIG model and the fluid-solid coupling model are applied to describe, respectively, the gas-liquid two phase flow, the bubble size distribution and interaction in the core and the heat transfer between cooling-pipe and liquid. The temperature distribution, velocity distribution, bubble diameter distribution in the nuclear reactor and heat transfer coefficient of the cooling coils were obtained by numerical method. The numerical results show that the temperature distribution in the core is more even when there is bubble disturbing, and the cooling pipe may achieve 85% heat transfer, which agrees with the test results. The study on the effect of different gas generation on the heat transfer of cooling pipe at nominal power shows that the heat transfer between the liquid and cooling pipe can be enhanced as the increase of bubble generation.