This　paper　investigates　the　seismic　performance　of　complex　comprehensive　interchange　underground　structures.　As　an　example,　the　seismic　performance　of　Nanjing　underground　bifurcated　tunnels　constructed　in　soft　soil　is　analyzed.　A　three-dimensional　(3D)　numerical　finite　element　analysis　model　is　established　considering　soiltunnel　interaction　to　analyze　the　seismic　response　characteristics　and　failure　mechanism　of　the　bifurcated　tunnels.　The　results　show　that　the　tunnel　damage　degree　is　closely　associated　with　the　deformation　response,　and　that　the　inter-story　drift　ratio　of　the　lower　layer　is　significantly　greater　than　that　of　the　upper　layer.　The　pushover　analysis　method　is　used　to　obtain　the　seismic　performance　curve　of　the　tunnel,　and　inter-story　drift　ratio　limits　are　defined.　The　IDA　procedure　is　then　used　to　conduct　multiple　non-linear　dynamic　time　history　analyses　based　on　the　3D　finite　element　model,　and　IDA　curve　clusters　are　drawn　with　peak　ground　acceleration　(PGA)　as　the　earthquake　intensity　measures　and　the　peak　inter-story　drift　ratio　as　the　damage　measures　of　tunnels.　The　corresponding　IDA　percentiles　lines　are　calculated,　and　the　seismic　fragility　curves　of　the　bifurcated　tunnels　are　established.　The　post-earthquake　functional　failure　probability　of　the　bifurcated　tunnels　is　obtained　under　different　seismic　fortification　performance　level.