The flow mixing phenomenon between subchannels in fuel assemblies has a direct impact on the CHF of the assemblies. To enhance the understanding of the flow mixing phenomenon, particularly the influence of local structures on flow mixing. This study combines CFD methods with subchannel analysis to study flow mixing under local structural. Based on the 5 × 5 rod bundle, establish a geometric model for the spacer grid, using CFX for calculation, and obtain the temperature distribution of subchannels at different axial positions under rated operating conditions. Using the subchannel analysis program to iteratively calculate the numerical simulation results, obtain the local mixing model of the spacer grid. The research reveals that local structures significantly affect flow mixing between rod bundle subchannels, with the scope of influence being related to the specific structure; the impact of spacer grids on mixing exhibits distinct local characteristics. Additionally, through data analysis, obtain the models of mixing coefficients of spacer grids. This study will help to improve the understanding of the flow mixing phenomenon. If the local mixing model is applied, the accuracy of thermal safety analysis for fuel assemblies will be enhanced.