Casing　shear　deformation　has　become　a　serious　problem　in　the　development　of　shale　gas　fields,　which　is　believed　to　be　related　to　fault　slipping　caused　by　multistage　fracturing,　and　the　evaluation　of　the　reduction　of　a　casing＇s　inner　diameter　is　key.　Although　many　fault　slipping　models　have　been　published,　most　of　them　have　not　taken　the　fluid-solid-heat　coupling　effect　into　account,　and　none　of　the　models　could　be　used　to　calculate　the　reduction　of　a　casing＇s　inner　diameter.　In　this　paper,　a　new　3D　finite　element　model　was　developed　to　simulate　the　progress　of　fault　slipping,　taking　the　fluid-solid-heat　coupling　effect　during　fracturing　into　account.　For　the　purpose　of　increasing　calculation　accuracy,　the　elastoplastic　constitutive　relations　of　materials　were　considered,　and　the　solid-shell　elements　technique　was　used.　The　reduction　of　the　casing＇s　inner　diameter　along　the　axis　was　calculated　and　the　calculation　results　were　compared　with　the　measurement　results　of　multi-finger　caliper　(MFC)　surveys.　A　sensitivity　analysis　was　conducted,　and　the　influences　of　slip　distance,　casing　internal　pressure,　thickness　of　production　and　intermediate　casing,　and　the　mechanical　parameters　of　cement　sheath　on　the　reduction　of　a　casing＇s　inner　diameter　in　the　deformed　segment　were　analyzed.　The　numerical　analysis　results　showed　that　decreasing　the　slip　distance,　maintaining　high　pressure,　decreasing　the　Poisson　ratio　of　cement　sheath,　and　increasing　casing　thickness　were　beneficial　to　protect　the　integrity　of　the　casing.　The　numerical　simulation　results　were　verified　by　comparison　to　the　shape　of　MFC　measurement　results,　and　had　an　accuracy　up　to　90.17%.　Results　from　this　study　are　expected　to　provide　a　better　understanding　of　casing　shear　deformation,　and　a　prediction　method　of　a　casing＇s　inner　diameter　after　fault　slipping　in　multistage　fracturing　wells.