Abstract: The third generation reactors have characteristics of safety and efficiency, and the C-shaped heat exchanger is the main equipment in the design of the third generation reactor, which is widely used in the design of the passive residual heat removal system. It is of great significance on reactor safety to study on the flow and heat transfer process of Cshaped heat exchanger. To study the coupling flow and heat transfer characteristics of its primary and secondary side, and also to obtain flow distribution mechanism in heat exchange pipes for optimizing structure design, a fullsize physical model of Cshaped heat exchanger was established in this paper. It included inletoutlethousings, inletoutlet pipes and 500 heat transfer pipes as the primary side, and the heat transfer model of the secondary side was also established. Numerical simulation studies under different thermal and geometric conditions were conducted by the software FLUENT to obtain the laws of temperature, speed, pressure and flow distribution between the heat transfer tubes in primary side. At the same time, the influence of inlet flow, inlet temperature, inlet position to the flow distribution and flow resistance distribution was analyzed, and the affecting mechanism of floating force and inertial force on flow distribution was also analyzed. The calculation results show that the current design of the heat transfer tube is uneven in flow distribution, the flow of inner side is low, the flow of outer side is high, and the inner layer has the phenomenon of reverse flow. The uneven flow distribution in the current design is mainly caused by thermosiphon (density difference causing gravity pressure difference). This phenomenon is more obvious when inlet flow is low or the inlet temperature is high. During the range of the thermal parameters in this paper, the phenomenon of the thermosiphon is always dominated in the influencing mechanisms of flow distribution. Three inlet pipe positions of 0° (existing design), 90° and 180° were compared and it is found that the position of the inlet pipeline is not obvious for flow unevenness. When the flow is low, the flow distribution unevenness of the existing design (overentering and downout) is optimal on the contrary. In further study, it is possible to explore measures to reduce flow distribution unevenness. The measures include changing the connection of the heat transfer tubes and the headers, changing the distribution of the heat transfer coefficient of the primary and secondary side, and changing the installation method of the inlet headers, etc.