Abstract: The natural circulation characteristics is the key operating characteristic of the primary circuit of the lead-based reactor, which has an important influence on the passive residual heat removal system of the reactor. The natural circulation characteristics and residual heat removal capacity in the reactor are an important part of the thermal hydraulic research of reactor. Using the porous media method, the 1/4 3D calculation model of the CiADS lead-based reactor was established, and the steady state calculation of rated conditions and low power operating condition were performed by FLUENT code. In order to study the transient process of residual heat removal of CiADS lead-based reactor under the station blackout accident, a 2D equivalent model which according to the principles of thermal hydraulic equivalence was established for transient calculation. The results show that CiADS lead-based reactor has low power natural circulation operation capability and a certain degree of accident tolerance; The results of 2D equivalent model and 3D model agree well with each other and the 2D model can be used in transient analysis; The 2D transient calculation results show that the passive residual heat removal system can cope well with the station blackout accident, and the CiADS lead-based reactor has good passive safety performance.