Abstract: Effect of late water injection on RCS repressurization during high pressure severe accident sequence in a typical PWR was analyzed.As the results shown,late water injection could increase RCS pres-sure when RPV failed without RCS passive depressurization.Especially in the condition of opening one PORV,RCS pressure could reach high pressure limit when RPV failed and the risk of HPME and DCH was dramatically increased.Integrity of containment could be threatened.However,in the condition of RCS pas-sive depressurization induced by pressurizer surge line creep failure,RCS pressure could be decreased to very low level even only one PORV was opened and two trains of emergency core cooling were implemented.The risk of HPME and DCH was eliminated.The more PORVs were opened,the faster accident progression was and the earlier RPV failed.RCS pressure was a little higher when PRV failed if two trains of emergency core cooling was implemented comparing with the condition with only one train of emergency core cooling.However the time of RPV failure was obviously delayed.From the point of delaying RPV failure and pre-venting containment early failure of view,the optimized late water injection was opening threc PORVs and implementing two trains of emergency core cooling.