Abstract: A physical model was built in this study to numerically investigate the heat transfer characteristics of supercritical pressure water within a square sub-channel by using the SSG Reynolds stress model. The corresponding fuel rod diameter was 8 mm and the pitch-to-diameter ratio was 1.2 respectively. The effects of pressure, mass velocity and heat flux on the heat transfer characteristics were analyzed in detail. It is found that the numerical results of the wall temperature trend are consistent with the experiment results. The peak of heat transfer coefficient decreases with the increase of pressure in the pseudo-critical region. The increase of mass velocity can significantly enhance the heat transfer in sub-channel in a wide enthalpy region. With the increase of heat flux, the wall temperature increases and the peak value of heat transfer coefficient decreases, gradually moving towards low enthalpy region at the same time.