In response to the increasingly prominent contribution of steam line break (SLB) accidents to the frequency of core damage in nuclear power plants, a third-generation pressurized water reactor nuclear power plant is taken as the object of study, adopts the research methodology of combining the deterministic and probabilistic (PSA) analysis with the risk-informed, carries out the sensitivity calculations of two conditions to definite the determination for two main steam lines breakage accidents caused by a large aircraft impact in an extreme case, obtains the minimum combination of PSA success criteria for the power plant system under the accident. The results show that there are more obvious differences in reactivity, primary pressure and aver-age temperature between Case 1 and Case 2 at 90 s after the accident, the sensitivity analysis indicates that the number of available train of the emergency feedwater system (ASG) is the reason for the difference in the calculation results between Case 1 and Case 2.The Core Damage Frequency(CDF) value of at least two ASG trains put in operation increased by 16.7% com-pared with at least one ASG train put in operation. When only 1 ASG is available and the Atmosphere Steam Dump System (VDA) is unavailable, the primary pressure will exceed the design pressure and the CDF value will increase by several orders of magnitude compared to the VDA availability. When two main steam lines break due to the aircraft crash, one ASG and the VDA is available to ensure the safety of the unit. Study results prove the reliability of a third generation unit and provide reference for PSA success criteria of multiple main steam lines break.