The　seismic　fragility　analysis　involves　employing　an　ensemble　of　ground　motion　records　to　establish　the　seismic　performance.　This　paper　investigates　the　effect　of　number　of　ground　motion　records　in　the　considered　ensemble　on　the　calculated　post-earthquake　failure　probability　of　subway　structure.　A　two-dimensional　(2D)　finite　element　model　was　established　considering　the　soil-subway　station　underground　structure　interaction.　The　cloud　and　incremental　dynamical　analysis　(IDA)　methods　are　employed　to　compute　the　seismic　demand　of　the　structure　and　the　seismic　fragility　curves　considering　different　ensembles　of　ground　motion　records.　The　results　revealed　that　the　number　of　records　have　a　significant　influence　on　the　structure　seismic　fragility　analysis　in　both　cloud　and　IDA　methods.　It　is　suggested　that　the　optimal　ensemble　size　should　comprise　300　ground　motion　records　in　the　cloud-based　seismic　fragility　analysis,　and　24　ground　motion　records　in　the　IDA　method.　The　results　also　showed　that　the　failure　probability　of　structure　predicted　by　the　cloud　method　is　greater　than　that　predicted　by　the　IDA　method.　These　findings　provide　practical　guidance　for　the　performance-based　seismic　design　of　underground　structures.