Abstract: With the rapid growth of natural gas demand in China, the safety, stability and timeliness of natural gas supply is much concerned, so it is of great importance to determine the reliability of natural gas pipeline network systems. In this paper, Shaanxi-Beijing natural gas pipeline network system was taken as the study object to conduct sensitivity analysis on the factors which have the most direct effect on the reliability of natural gas pipeline network system. The sensitivity analysis was carried out based on the typical operational mode by using the Six-Sigma rule which is commonly used in quality engineering. And the most direct influential factors of 5 aspects include distribution plan along the route, pipe section and station reliability and market demand. It is indicated that the natural gas pipeline network system has a certain design redundancy and high reliability, so it is less affected by the reliability fluctuation of its own factors but more by the demand fluctuation of gas supply. The research results provide the theoretical support for improving the gas supply guarantee capacity of natural gas pipeline network systems. (6 Figures, 8 Tables, 21 References)