Computational fluid dynamics (CFD) research of flow boiling and heat transfer in single rod channel is carried out based on RPI wall boiling model by CFD code Fluent. The numerical model is validated with published experimental date. According to numerical results, it can be seen that the numerical model is able to calculate the cross sectional averaged void fraction in a vertical tube. The visualization experiments of flow boiling will be performed in the near future to investigate the boiling characteristics of a vertical square channel with one heating rod. However, due to the limitations of measuring methods, visualization experiments can only be performed to obtain local visualization data, but local flow field characteristic parameters are difficult to obtain. Therefore, in order to deeply understand the boiling mechanism, the local flow characteristics including the distribution of void fraction, liquid temperature as well as velocities, a numerical study are performed to investigate the characteristics mentioned above, and detailed analysis are presented in this paper. In this study, the heat flux is the possible critical heat flux under corresponding working condition based on Bowring correlation; and the purpose of this work is focusing on the flow boiling and heat transfer when the heat flux is approaching critical heat flux. This work is meaningful for the subsequent integration on the mechanism of boiling two-phase flow.