Abstract: The leakage of the pressure boundary of the nuclear power plant is an important operation event. The response and treatment after the occurrence of abnormal events is focus on the safety management of the nuclear power plant. Based on the treatment of the leak point on the extension branch pipe of the pressure boundary of 300 MW Qinshan Nuclear Power Plant,combined with the evolution of the process parameters of the unit, the ideas and principles to find the leak point are analyzed, and the response and treatment of the similar leakage in nuclear power plants are analyzed. At the same time, according to the method of root cause analysis, combined with the requirements of the American Society of Mechanical Engineers（ASME） specifications,this paper analyzes the scheme, basis and main points of solving this problem, and puts forward the subsequent improvement suggestions for the design and the daily in-service inspection. Especially the method to effectively manage the small branch pipe is proposed to ensure the safety of the nuclear power plant. It can provide a good reference for the management of pressure boundary for small branch pipes in domestic nuclear power plants.