Casing　pressure　test　is　an　important　link　of　the　oil　and　gas　well　cementing,　but　excessive　casing　pressure　test　may　cause　stress　failure　or　plastically　deformation　of　the　cement　sheath,　and　generate　micro-annulus　on　the　casing-cement　sheath　cemented　surface,　then　lose　seal　integrity　of　the　cement　sheath.　According　to　the　basic　theory　of　elastoplastic　mechanics　and　considering　the　volume　invariance　of　plastic　strain　and　the　influence　of　elastic　strain　on　volume　change,　the　Mohr-Coulomb　criterion　was　used　to　establish　the　casing-cement　sheath-surrounding　rock　combination　model.　The　formula　for　calculating　the　micro-annulus　of　the　casing　pressure　test　was　derived,　the　influence　of　the　internal　pressure　of　the　casing　during　the　pressure　test　and　the　elastic　modulus　of　the　cement　sheath　on　the　radial　contact　stress　of　the　cement　sheath　cementation　interface　analyzed　after　the　cementing　operation　was　completed.　The　results　showed　that:　(1)　The　generation　of　micro-annulus　of　casing　pressure　test　was　determined　by　the　pressure　increase　process　and　the　pressure　relief　process.　The　pressure　increase　process　may　lead　the　cement　sheath　into　plasticity,　and　the　radial　stress　at　the　interface　turns　into　tensile　stress　and　the　micro-annulus　could　appear　at　the　first　interface　by　the　decrease　of　internal　pressure　during　pressure　relief　process.　(2)　The　varying　internal　pressure　had　a　great　influence　on　the　fatigue　failure　of　the　cement　sheath,　so　the　continuous　multiple　casing　pressure　test　should　be　avoided　after　the　cementing　was　completed.　(3)　Under　the　condition　of　maintaining　the　integrity　of　the　cement　sheath,　increasing　the　tensile　strength　of　the　cement　stone　and　reducing　the　elastic　modulus　of　the　cement　stone　could　improve　the　pressure　bearing　capacity　of　the　first　interface　during　the　casing　pressure　test.　The　model　can　provide　a　theoretical　basis　for　the　mechanical　parameter　design　of　the　cement　sheath,　and　provide　guidance　for　on-site　construction　to　reduce　or　avoid　the　risk　of　failure　of　the　first　interface　seal　integrity　for　the　casing　pressure　test.　©　2019　Trans　Tech　Publications,　Switzerland.