Sustained　Casing　Pressure　(SCP)　has　become　a　serious　problem　in　the　development　of　shale　gas　fields,　which　is　believed　to　relate　to　the　multistage　fracturing.　However,　few　researches　were　on　this,　especially　the　study　of　micro-annulus　at　the　interface　between　the　cement　sheath　and　casing,　which　was　one　of　the　mechanisms　of　the　cement　sheath　integrity　failure,　and　always　caused　by　accumulated　plastic　deformation.　This　paper　presented　experimental　and　numerical　investigations　of　the　emergence　and　development　of　plastic　deformation　occurred　at　the　interface　between　the　cement　sheath　and　casing　during　multistage　fracturing.　Mechanical　tests　and　physical　simulation　experiments　were　carried　out　so　as　to　analyze　the　plastic　deformation　variation　characteristic　of　the　cement　sheath　under　cyclical　loading　and　unloading.　3-D　numerical　models　were　established　to　analyze　the　emergence　and　development　of　the　accumulated　plastic　deformation　based　on　the　test　results,　considering　the　actual　engineering　and　geological　conditions　during　multistage　fracturing　in　shale　gas　wells,　which　presented　a　new　method　to　calculate　the　width　of　the　micro-annulus　between　the　cement　sheath　and　casing.　The　research　showed　that　the　numerical　results　were　in　accordance　with　the　experimental　results.　With　the　increase　of　cycle　times,　the　accumulated　plastic　deformation　increased,　which　was　the　greatest　for　the　first　time,　and　then　increased　linearly.　Decreasing　elastic　modulus　and　increasing　Poisson＇s　ratio,　cohesive　force　and　internal　friction　angle　could　facilitate　the　protection　of　the　cement　sheath　integrity.　Finally,　a　new　cement　slurry　system　was　proposed　based　on　the　results　of　study　and　applied　in　5　wells　with　good　effects　acquired,　none　of　them　showed　SCP　after　fracturing.