The　sustained　casing　pressure　(SCP)　occurring　during　multistage　fracturing　in　shale　gas　wells　is　an　important　problem　to　solve.　This　paper　presents　experimental　and　numerical　investigations　of　the　emergence　and　development　of　the　micro-annulus　generated　by　the　cumulative　plastic　strain.　A　series　of　mechanical　tests　were　conducted　to　quantify　the　stress-strain　relationship　and　obtain　the　fundamental　mechanical　parameters　of　the　set　cement.　A　new　full-size　physical　simulation　experimental　facility　was　built　to　identify　the　major　controlling　factor　under　cyclic　loading-unloading.　The　experimental　results　demonstrated　that　a　cumulative　plastic　strain　was　generated　during　the　cyclic　loading-unloading.　Micro-annulus　emerged　in　the　first　interface　and　became　the　pathway　for　gas　migration.　A　set　of　three-dimensional　numerical　models　was　developed　to　analyze　the　micro-annulus　emergence　and　development,　considering　the　non-uniform　in　situ　stress,　fracturing　pressure,　and　other　actual　geological　parameters.　Sensitivity　analysis　was　conducted　to　analyze　the　variation　in　the　micro-annulus　in　the　horizontal,　inclined,　and　vertical　segments.　The　numerical　results　were　consistent　with　the　experimental　results　and　demonstrated　that　decreasing　the　elastic　modulus　and　increasing　the　Poisson＇s　ratio,　cohesive　strength,　and　internal　friction　angle　were　beneficial　for　protecting　the　integrity　of　the　cement　sheath.