Reliability assessment of electricity-gas integrated energy system is undertaken via a new analytical method in this paper. Firstly, reliability models of coupling devices are developed to quantify interdependences between power distribution system and gas distribution system. Afterwards, reliability is evaluated separately in the two subsystems considering their distinctions in operation characteristics. A new topology simplification approach is presented to calculate reliability indices of natural gas system while reliability of power distribution system is evaluated based on the shortest path method. Finally, a practical case combined a 10 kV feeder of distribution station in Guangdong with Belgain natural gas network is exploited to verify the effectiveness and practicability of the proposed method.