Abstract: Taking the fuel assembly of thermal spectrum supercritical water-cooled reactor（SCWR） as the research object,and on the condition of average orifice size,the reactor core power distribution is simulated,and the thermal hydraulic calculation model is established and the corresponding program is developed.The coolant mass flux distribution and related parameters distributions in the parallel channels is calculated.The results show that the axial density distribution and distribution core power of each fuel assembly group channels is very inhomogeneous,causing a large mass flux difference,which could be resolved through in-creasing the orifice size of high power fuel assembly groups.