Abstract: In this study, the 3×3 rod bundles simulation specimen was modeled and the convective heat transfer characteristics in steam cooling condition were analyzed with CFD tools. Results show that the wall heat flux distribution along the circumferential direction of the rod bundle channel is obviously non-uniform, which means that the fluid-solid coupling method is superior to the uniform heat flux method in simulating details of heat transfer. The distributions of thermal parameters, such as steam velocity field, temperature field, heat flux and heat transfer coefficient, are affected by entrance effect, wall effect, heat source distribution and physical properties. Pressure increasing and hydrogen enhances the heat transfer capability in the rod bundles channel. The variation tendency of heat transfer coefficient along the axial direction in the heating-up section agreed well with Deissier in Reference 13. The heat transfer coefficient agreed well with the related formula in WCOBRA/TRAC code. The simulating method in this study is reasonable, and the calculation results can provide a technology support for the experimental simulator design in the future.