Abstract: Based on the preliminary design passive residual heat removal system project of the modular reactor, and considering the structure and operation characteristics, the reasonable control volumes are divided and the mathematical models are established. Using numerical iteration method and in common currency program, the transient-state codes is adopted to analyze the thermo-hydraulic characteristics of the passive residual heat removal system. The results show that, after the reactor power failure, natural circulation of system can quickly be established; in the passive residual heat removal process, no flow instability happens; emergency cooler heat exchange area change of residual heat removal capability has no significant effect in a certain range.