Abstract: The flow and heat transfer characteristics in the fuel assembly of the reactor core bundle affect the relevant thermal parameters of the reactor core, and the critical heat flux (CHF) and departure from nucleate boiling ratio (DNBR) are increased due to turbulent mixing between subchannels. The turbulent mixing effect between sub-channels is a comprehensive mixing effect caused by multiple factors (pressure difference between channels, geometry, mechanical components, etc.). The mixing between sub-channels can be divided into natural mixing and forced mixing. Cross-flow mixing, as one of natural mixing, is a directional flow of coolant through the gap between sub-channels along the pressure gradient due to the pressure difference between adjacent subchannels. Cross-flow mixing is a type of cross-flow mixing with a net mass transfer, which is also accompanied by an exchange of momentum and energy between the coolants. Another kind of natural mixing is turbulent mixing, which is because the fluid micelle itself is in a state of turbulent pulsation, and at the position of the gap between adjacent subchannels, the fluid micelle forms a vortex flow due to turbulent pulsation, resulting in the exchange of coolants on both sides of the gap. However, due to the vortex flow, there is no net mass transfer of the coolant between the subchannels. In order to improve the economy and safety of the nuclear reactor system, this paper intends to use the computational fluid dynamics (CFD) method to study the turbulent mixing effect between the rod bundle subchannels. In this study, the subchannel was modeled, the SST k-ω model was selected for calculation, and the grid sensitivity analysis was completed. It is found by calculation that when the Reynolds number is small, the single-phase turbulent mixing coefficient increases with the Reynolds number, and when the Reynolds number reaches a certain value, the single-phase turbulent mixing coefficient is approximately constant. At the same time, the turbulent mixing coefficients of subchannels can be accurately calculated by the analog concentration calculation method and the gap turbulent heat flow method, and there is no significant difference in the axial distribution of the calculated turbulent mixing coefficients.