Quantification　of　structural　damage　states　in　the　fragility　analysis　and　seismic　performance　evaluation　of　underground　structures　usually　adopts　the　deterministic　threshold　values,　which　is　a　limitation　in　the　framework　of　performance-based　seismic　engineering.　A　new　seismic　fragility　analysis　method　considering　the　fuzzy　probability　of　damage　states　of　the　underground　structures　is　proposed　based　on　the　fragility　analysis　framework　of　the　traditional　underground　structures.　A　two-dimensional　nonlinear　finite　element　model　for　a　three-story　and　three-span　subway　station　structure　is　established　fully　considering　the　soil-structure　interaction　(SSI).　The　input　ground　motions　used　in　the　nonlinear　incremental　dynamic　analysis　of　the　SSI　model　are　back-calculated　from　an　ensemble　of　21　actual　earthquake　records　on　the　ground　surfaces.　The　triangle　and　quasi-normal　membership　functions　are　introduced　to　compute　the　membership　degrees　of　the　maximum　interlayer　drift　ratios　for　different　damage　states　so　as　to　derive　the　fuzzy　failure　probability　under　different　intensity　levels　of　ground　motions.　The　maximum　likelihood　estimation　method　is　used　to　fit　the　numerical　results　and　establish　the　seismic　fragility　curves　of　subway　station　structures　considering　the　fuzzy　probability　of　damage　states.　The　results　of　the　numerical　study　indicate　that　the　seismic　fragility　curves　of　the　underground　structures　derived　from　the　proposed　method　generally　give　a　safer　estimation　of　the　seismic　performance　of　the　structures.　The　dispersions　of　the　seismic　fragility　curves　increase　with　the　consideration　of　the　fuzzy　probability　of　damage　states.　Besides,　the　influences　of　the　membership　function　types　on　the　results　of　the　fuzzy　seismic　fragility　analysis　can　be　ignored.　©　2022　Chinese　Society　of　Civil　Engineering.　All　rights　reserved.