Based　on　a　practical　engineering　case　of　seismic　strengthening,　this　paper　used　the　enlarging　cross-section　method　and　an　external　self-centering　substructure　to　improve　the　seismic　performance　and　seismic　resilience　of　existing　frame　structures.　Among　them,　the　external　self-centering　substructure　included　setting　a　self-centering　precast　beam　and　diagonal　braces.　Utilizing　the　OpenSees　finite　element　platform,　a　seismic　fragility　analysis　was　carried　out　to　compare　the　improvements　in　seismic　performance　and　seismic　resilience　before　and　after　strengthening.　The　analysis　results　show　that　the　proposed　modelling　method　could　be　simulated　satisfactorily.　The　maximum　inter-story　drift　and　the　residual　inter-story　drift　of　the　strengthened　frame　structures　decreased　significantly　under　the　same　peak　ground　acceleration.　The　peak　ground　acceleration　of　the　strengthened　frame　structures　significantly　increased　under　different　performance　levels.　Additionally,　the　exceedance　probability　of　the　strengthened　frame　structures　was　obviously　reduced,　which　reflected　that　the　seismic　performance　and　seismic　resilience　of　the　strengthened　frame　structures　were　significantly　improved.