The rod bundle channel, as a common structure in nuclear reactor core fuel assemblies, features open channel units and mixing vanes that induce significant lateral flow and turbulent mixing between adjacent fuel assembly channels. Understanding the lateral flow between adjacent channels is crucial for improving the comprehension of thermal–hydraulic mechanisms within the reactor core and enhancing the safety and efficiency of nuclear reactors. In this study, a computational analysis method based on the fluid network approach is developed for a typical rod bundle channel fuel assembly system. The method simplifies the flow system of the rod bundle fuel assembly into a fluid network structure composed of node elements and channel elements. The results from fluid network calculations are coupled with Computational Fluid Dynamics (CFD) calculations to achieve the purpose of calculating the lateral flow between adjacent fuel assembly channels. By exchanging boundary conditions between the fluid network model and CFD simulations, the flow rates in the channel elements and the pressures at the node elements were obtained. This study demonstrates that the proposed method can be employed for coolant flow field calculations and lays a theoretical foundation for subsequent fluid network method calculations of the entire reactor core flow field.