Abstract: In the external reactor vessel passive cooling system, the refueling water pool is used as the heat trap. Driven by buoyancy force, thermal stratification phenomena occur spontaneously in the pool. The thermal stratification and mixing phenomena, development process and main influencing factors in the refueling water pool with the REPEC test facility were studied. The experimental results show that the temperature distribution in the experimental water tank varies with height, the circulation flow is one of the most important factors which influence the thermal stratification process in the water tank, and the effect of initial water temperature is negligible. The analysis result of the 1-D non-dimensional transient temperature field equation shows that the development of the temperature field in the water tank is mainly controlled by convective heat transfer.