The study of thermal characteristics during startup is one of the most important aspects for safety analysis of supercritical water-cooled reactor (SCWR). According to the given sliding pressure mode of SCWR, thermal analysis on temperature-raising phase and power-raising phase of startup are carried out. Considering the radial heterogeneity of power distribution, thermal characteristics for different assemblies during startup are also put forward. The results show that, during temperature-raising phase with core power increased only, the temperature of moderator, coolant and fuel cladding in inner assemblies are increased with little amplitude. During power-raising phase with core power and feed-water flow rate increased, the coolant temperature keeps unchanged, but the moderator temperature is decreased. With a greater variation of power, fuel cladding temperature shows a greater increase. Furthermore, considering the uneven distribution of radial power, thermo-hydraulic characteristics with uneven cladding temperature distribution shows a certain horizontal heterogeneity for different fuel assemblies, which becomes serious as flow rate and power increase. By adjusting flow rate distribution in different fuel assemblies or changing power setting during startup, the cladding temperature difference could be effectively reduced, which provides a certain reference for startup optimization of SCWR.