Abstract: The RPI （Rensselaer Polytechnic Institute） wall boiling model, embedded in the context of two-fluid model of Fluent14.5, was applied to simulate subcooled boiling in core fuel bundle channel with mixing vanes and space grid. The subcooled boiling key parameters including flow field, temperature field and void fraction distribution were obtained. The analysis of the effect of mixing vanes and space grid on flow channel thermal-hydraulic characteristics was conducted. It is shown that the existence of space grid can cause a large pressure drop, and mixing vanes have a great influence on the key parameters of flow field and temperature field. The results of RPI model show a good agreement with Bartolemei experiment data.