Abstract: The physical model of the sub-channel within a triangular rod bundle, of which the rod diameter is 8 mm and the pitch-to-diameter ratio is 1.4, was built to numerically investigate the flow and heat transfer characteristics of supercritical water in the sub-channels of supercritical water-cooled reactor. The numerical investigation was performed within the range of pressure from 23 to 28 MPa, mass fluxes from 700 to 1300 kg/(m²•s), and heat fluxes from 200 to 1 000 kW/m². By using the Reynolds stress turbulence model SSG, the effects of mass flux, pressure and heat flux on the flow and heat transfer characteristics were analyzed in detail, as well as the secondary flow characteristics in different enthalpy regions. The results show that the numerical results agree well with experimental results. The higher the mass flux is, the better the heat transfer performs. In the pseudo-critical enthalpy region, the peak of heat transfer coefficient decreases greatly with the increase of pressure and heat flux. The high heat flux induces high inner wall temperature and might cause the heat transfer deterioration. The obvious secondary flow is found in the direction perpendicular to the main flow, and six symmetrical vortices are formed in the cross section of sub-channel. The velocity of the secondary flow reaches a peak near the narrow gap. Although the structures of the secondary flow are similar in different enthalpy regions, the secondary flow velocity increases with bulk enthalpy.