Heat-transfer characteristics are especially significant among all issues for the supercritical water-cooled reactors (SCWRs). The two-peak wall-temperature phenomenon that could be verified by Shitsman and Jackson’s experiments occurred in the regions of for vertical upward flow under the extreme heating conditions or accident cases (i.e., for water). However, so far the special two-peak result has not been paid much attention due to the difficulty of the experiments. Hence, in this study, numerical analysis was carried out to investigate the characteristics of heat-transfer deterioration (HTD) two-peak phenomenon of supercritical pressure water/carbon dioxide in the conditions of high . The results showed that the higher the heat fluxes were, the more temperature peaks might appear and more unstable flow might be presented. Finally, the mechanism of two-peak HTD was studied through quantitatively analyzing the distribution of flow parameters and thermophysical properties in the near-wall region.