Abstract: Mixing of hot and cool coolants during high pressure safety injection in pressurized water reactors (PWRs) can lead to pressurized thermal shock, which results in aging of the pressure vessel. Based on the ROCOM experimental database, the mixing of hot and cool coolants with density difference was numerically simulated by using CFD method, and the simulation results were validated and analyzed. The results show that the force convection mixing and buoyancy-driving mixing are main reason for mixing of liquid in cold leg and downcomer. During coolant injection from one of the loops, the injected flow could product backflow from the lower plenum to the downcomer, which can affect the mixing characteristics of coolant.