In order to study the safety of pipeline operation under the design coefficient of 0.8, the limit state equation is established for X80 pipelines which are most widely used in West-to-East pipeline project, and the reliability of pipeline is analyzed by Monte-Carlo method, and the utmost gas transmission pressure and the design coefficient are calculated according to the target reliability specified by APl579 Fitness-For-Service. The results show that when the coefficient of variation is 0.1, the design coefficient is only increased to 0.8 and the reliability of pipeline operation is less than the target reliability of 0.99; if the pressure fluctuation of the pipeline is reduced to 0.08, the reliability of the pipeline can reach the required value. At last, the design coefficients under different variable pressure coefficients were calculated to provide basis for future research.