A natural gas pipeline network system is an example of large-scale infrastructure. It is composed of a number of long-distance natural gas transmission pipelines and their supporting facilities such as underground gas storages, with information and natural gas interconnection capabilities. As critical infrastructure connecting gas resources and markets, the security of gas supply is directly related to the safe and reliable operation of the natural gas pipeline network system. Therefore,it is necessary to study the reliability of natural gas pipeline systems to ensure public safety and energy security in China. An objective reliability index system is the key to quantifying the reliability of the natural gas pipeline network system. In this paper,by reviewing previous research into natural gas pipeline network reliability and combining the practical application requirements of the pipeline network reliability research, the reliability of natural gas pipeline network systems is divided into mechanical reliability, hydraulic reliability, and gas supply reliability based on the characteristics and functions of the natural gas pipeline network. Moreover, three types of reliability evaluation indicators are proposed, and are used to quantify the capacity of normal operation, the capacity of completing the required gas supply tasks, and the capacity of satisfying required demand.