Abstract: To improve the accuracy of the numerical simulations using RPI (Rsensselaer Polytechnic Institute) subcooled boiling model in rod bundle and provide some references for choosing sub-model parameters in RPI model, three important sub-models (bubble departure diameter, wall nucleation site density and bubble detachment frequency) and two important non-drag force models (lift force and turbulent dispersion force) were investigated by comparing simulation results of void fraction and wall superheat to the FT-6a experiment data. The analysis results show that the sub-models in RPI model exert complicated effects on the simulation results of void fraction and wall superheat. The simulation results should be validated and the optimal sub-model parameters should be determined by comparing more than one simulation data to the experiment data rather than a single data result. The non-drag forces play an important role in the radial distribution of void fraction in rod bundle. The life force has the effect of restraining the bubble away from the heated wall and the turbulent dispersion force has the effect of promoting the bubble into the main flow region.