Abstract: This paper analyzes the main structural and mechanical parameters of X80 pipeline, including the yield strengthof base metal, tensile strength of base metal, weld tensile strength, impact energy of base metal, weld impact energy, impactenergy of heat-affected zone (HAZ) and pipeline pressure. All these parameters follow a normal distribution and their meanvalues and standard deviations are given. Weibull distribution and lognormal distribution methods are adopted to calculatethe defect distribution. Based on the FAD technology and Monte-Carlo simulation method and applying foreign targetreliability indices, the reliability of the long-distance X80 gas pipeline with defects is assessed. Results show that even in thepresence of defects undetected, the reliability of X80 gas pipeline with defects can fully meet standard requirements. Themethod is suitable for the safety assessment of gas pipelines and has practical value.