Abstract: The passive residual heat removal (PRHR) system plays an important role in the safe operation of the reactor. However, studies have shown that the bypass flow in the primary system increases the flow rate of reactor core, which increases the flow resistance and enhances the heat transfer in the core. Therefore, operation characteristics of the system become complicated, and the effect of the bypass flow needs to be analyzed. Based on the one-dimensional N-S equation, the operational characteristic analysis model of the PRHR system considering the bypass flow was established and verified. Then the mechanism of the bypass flow was revealed, and its influence on the operating characteristics of the PRHR system was analyzed. The results show that the existence of bypass flow reduces the heat-removal capacity of the PRHR system, which affects the passive safety of the reactor. The influence of bypass flow needs to be considered in the design and safety analysis of the reactor PRHR system.