Abstract: The CFD model of CARR main body was set up in this paper. The CFD model was verified by comparing the engineering experimental data and the simulation results of forced circulation steady condition and natural circulation transient condition, and this model was also used to simulate the natural circulation capability test condition. Through simulating the operation mode and test mode by using this CFD model, the variations of flow and temperature distributions during the transient procedure from forced circulation to natural circulation were achieved, and the natural circulation characteristics of CARR were analyzed and studied. Many reactor safety analysis jobs of CARR could be done based on this CFD model in the future, for example, test project verification, accident analysis, and so on.