Abstract: In order to analyze the thermal and hydraulic characteristics of the SBO and loss of ASG accident of CPR1000 reactor, the RELAP5 program is used to model and calculate the reactor core and the NSSS system of the reference unit, and the thermal and hydraulic characteristics of the whole accident process are analyzed combined with the calculation results. The results show that the thermal hydraulic characteristics of the primary loop in this accident are more complicated. During the accident, the primary loop has successively experienced the emptying state of the pressure vessel, the emptying state of the U pipe of the steam generator, the emptying state of the cold and hot legs, and the partially exposed and fully exposed state of the core, but the coolant in the PZR is the last to be discharged, which may lead to severe accident consequences of core melting and high-pressure melting ejection. In addition, the improvement suggestions that is more beneficial to the core safety is put forward after comparing and analyzing the intervention actions and mitigation measures combined with the current nuclear power plant accident regulations.