This paper analyzes the leakage of seawater pipeline system in a nuclear power plant during commission-ing. From the construction to the commissioning process,the multi factor exclusion method is adopted to analyze the corr-osion problem. Through the analysis of material properties,stress distribution and medium environment of materials,it is concluded that cracks and corrosion are stress corrosion,which meet the three elements of stress corrosion:the material is analyzed and found to be sensitive material;the material and structure used in some areas of the system are different,which leads to the effect of structural stress and load stress;it operates in corrosive medium seawater Therefore,the condition of stress corrosion cracking is achieved,and the medium leakage is caused by the occurrence of penetrating cracks. The analysis process of the crack and corrosion can be used for reference.