The higher incident radiation flux and the higher thermal to electric efficiency lead the solar tower power with supercritical water as heat transport fluid in central receiver to be one of the most promising solar thermal power. In this paper, heat transfer of supercritical water in a vertical tube of the solar tower receiver has been investigated using Computational Fluid Dynamics (CFD) method. A 3D simulation model is employed to really express distribution of heat flux in the circumferential direction of a circular tube heated by incident solar flux on one side. The RNG k-ε model with the standard wall function is employed to carry out numerical simulations and the results agree with experimental data well. The simulations reveal that the average heat transfer coefficient depends strongly upon the mass flux and incident solar flux. At sufficiently high mass fluxes, the average heat transfer coefficient reaches the peak value at a bulk temperature slightly less than the pseudo-critical temperature. At low mass flux relatively to the incident flux, heat transfer deterioration occurs in the pseudo-critical region. The buoyancy effect which is important factor causing heat transfer deterioration in supercritical water is obviously weakened in downward flow, even can be negligible. and h/h0 are modified to a limit of 10-6 for the onset of mixed convection and 0.45 for the onset of heat transfer deterioration when supercritical water in circular tube is heated by incident flux. And, the relations between the limit incident radiation flux above which the heat transfer deterioration occurs and mass flux for the supercritical water solar tower receiver is given as qin=0.453G