Abstract: Taking a nuclear power plant as an example, the structural integrity of the nuclear safety class 1 pipe is analyzed and evaluated in this paper, and the design margin of the pipeline is analyzed according to the standard. The structural strength of pipeline is evaluated according to the standard, thermal ratchet is evaluated by analyzing the pipeline temperature theoretically, and the fatigue life of the pipeline is evaluated by using the simplified rain flow method. The results show that the minimum wall thickness of the main coolant pipe meets the requirements of the standards when reduced to 55 mm, though the safety margin is small. The safety margin of fatigue and thermal ratcheting evaluation of branch is minimum.