Casing　shear　deformation　has　become　a　serious　problem　in　the　development　of　shale　gas　fields,　which　is　believed　to　relate　to　fault　slipping　caused　by　multistage　fracturing.　This　paper　presents　a　new　investigation　aiming　at　understanding　the　reduction　of　casing　inner　diameter　after　fault　slipping　under　different　conditions,　according　to　microseism　data　and　calliper　surveys.　A　mathematical　model,　which　was　verified　by　measurement　results,　was　established　to　build　a　relationship　between　microseism　moment　magnitude　and　slip　distance.　A　3D　numerical　model,　which　was　verified　by　physical　model　experiments,　was　developed　to　quantify　the　relationships　between　slip　distance　and　the　reduction　of　casing　inner　diameter.　And　then　the　relationships　among　microseism　moment　magnitude,　slip　distance　and　the　reduction　of　casing　inner　diameter　were　established.　The　influence　of　casing　internal　pressure,　slip　distance,　mechanical　parameters　of　cement　sheath,　and　casing　thickness　on　the　reduction　of　casing　inner　diameter　was　analyzed.　The　mathematical　calculation　results　showed　that　with　the　increase　of　the　degree　of　moment　magnitude,　the　radius　and　slip　distance　increase,　and　with　the　increase　of　stress　drop,　the　radius　decreases　and　the　slip　distance　increases.　The　numerical　analysis　results　showed　maintaining　high　pressure,　increasing　the　elasticity　modulus　and　decreasing　the　Passion　ratio　of　cement　sheath　and　increasing　the　casing　thickness　were　beneficial　to　decrease　the　reduction　of　casing　inner　diameter.　Keeping　the　designed　horizontal　segment　of　well　trajectory　away　from　the　fracture-developed　area,　or　be　parallel　to　natural　fracture　can　decrease　the　slip　distance　of　fault,　which　can　decrease　the　reduction　of　casing　inner　diameter.　Finally,　some　effective　measures　to　reduce　or　prevent　casing　shear　deformation　were　recommended.