Abstract: In order to realize more objective quantitative risk assessment of natural gas pipeline network systems, this paper takes the natural gas pipeline network systems in Beijing as a case, thoroughly analyzes the possible factors causing pipeline network accidents, statistically analyzes the accidents, and establishes a risk assessment index system for natural gas pipeline network systems. In accident probability assessment, the accident probability is modified based on reliability analysis on pipelines and pipeline networks and integrated with designed parameters of natural gas pipeline networks and the characteristics of the surrounding environments, and in consequences severity assessment, risk tolerance and control capacity of pipeline networks accidents is included, vulnerability analysis of natural gas pipeline networks is finished, and correction of consequence severity is finally made possible. After accident probability and consequence severity is classified, the risk level of a natural gas pipeline networks is calculable relying on risk matrix.