Abstract: In order to improve the safety of reactors, 15% safety margin requirement is proposed by NPP utility. To improve the large break loss of coolant accident (LBLOCA) safety margin of CPR1000 NPP, two measures were proposed considering the minimum change and the maximum benefit, including increasing the water volume in the accumulators (ACCs) and using hot rod statistical treatment method (HRSM). Based on CATHARE program, the sensitivity calculation was carried out for the increase of the water volume in the ACCs, so the optimal water volume was got. In HRSM, three input parameters which greatly affected the calculation results were sampled, and the calculation results were statistically analyzed, so the 95%95% value (95% probability value under 95% confidence level) was got. The results show that the LBLOCA safety margin of CPR1000 NPP increases by 6.5% after using the two measures.