Abstract: To investigate the startup characteristics of whole supercritical pressure water cooled reactor (SCWR) system, the complete startup system model of the SCWR was established with the analysis code SCTRAN, based on the CSR1000 core, high performance light water reactor (HPLWR) steam cycle and SCWR circulation startup loop. The correctness of the model was verified in comparison with the steady-state parameters of the steam cycle of the HPLWR. A sliding pressure startup procedure with the circulation loop that employs a control system was analyzed, and the transient performances of the core, steam drum, steam turbine, reheaters, steam extraction, and heaters at each stage were obtained. The calculation results show that the startup sequence and startup thermal parameters agree well with the expectation: the system starts up stably and the core remains in the single phase; the inlet steam of the turbine stays supercritical; the core inlet temperature can reach 280℃ after the high-pressure and low-pressure heaters; the inlet pressure of the high-pressure turbine can be kept constant. During the startup procedure, the maximum cladding surface temperature remains below the limit temperature of 650℃. The entire startup procedure is safe and reliable.