In this study, computational fluid dynamics simulations are carried out to predict the thermal-hydraulic behavior of supercritical fluids in the subchannel of supercritical water-cooled reactor (SCWR) fuel channels. The thermal-hydraulic behavior of supercritical water in triangular array and square array fuel rod bundles is studied numerically. The effects of various parameters including the pitch-to-diameter ratio and Reynolds number on the flow and the heat transfer characteristics are investigated. It is found that the turbulent mixing coefficient of supercritical water in subchannels is strongly dependent on the fluid bulk temperature and pitch-to-diameter ratio in the vicinity of the pseudo-critical point. To have a higher overall turbulent mixing coefficient, a pitch-to-diameter ratio less than 1.2 is recommended for the design of SCWR. The turbulent mixing coefficient correlation for the triangular array rod bundle is developed in this study based on the numerical results. However, the correlation for the mixing coefficient for the square array rod bundle cannot be expressed as a general correlation.