As one of the lifeline projects, an urban gas network is a complex system, as it requires maintenance of the supply capacity when any single pipeline is isolated due to failure. For such a system, its reliability needs to be evaluated. Considering that existing structural reliability and hydraulic reliability analyses reflect different aspects of the working conditions of an urban gas network, system reliability theory is employed to explain that only the gas supply reliability can achieve a comprehensive evaluation of the work capacity of the entire urban gas network, as it takes into account the combined influence of the structural reliability and hydraulic reliability. To calculate the parameters in the gas supply reliability evaluation, such as pipeline failure rate, flow reduction in the gas network under different failure conditions, etc., some research achievements in the field of structural reliability and hydraulic reliability are fully utilized. Then, the detailed calculation procedures of these parameters are given to evaluate the gas supply reliability in terms of operational and practical considerations. Finally, using an example of a simple double-loop gas network, the detailed process of the gas supply reliability evaluation of an urban gas network is described, and the feasibility of this evaluation method is also illustrated.