The Steam Generator Tube Rupture accident is the Design Basis Condition-3 of PWR nuclear power plant. Compared with other design basis accidents, the frequency of Steam Generator Tube Rupture is relatively high, and the Steam Generator Tube Rupture is highly concerned in engineering design. It is clearly regulated in HAD 102 that Single Failure must be taken into ac-count in the accident analysis of Design Basis Condition-3 and Design Basis Condition-4. In this paper, the research is carried out on the Generation III Pressurized Water Reactor. Firstly, the safety system for mitigating the Steam Generator Tube Rupture accident is sort outed. Then, the Single Failure hypothesis is assumed based on the safety system for mitigating the Steam Generator Tube Rupture accident. The Single Failure hypothesis is analysed by quantitative calculation and quanlitative analysis. The results show that the single failure of the main steam isolation valve is the worst one, and the accident consequences still meet the acceptance criteria.