This　study　aims　to　investigate　the　true　stress　and　the　plastic　shakedown　behavior　of　a　cyclically　loaded　pressure　vessel.　To　serve　this　study,　an　experimental　platform　has　been　built　up,　which　comprised　of　a　self-made　thin-wall　cylinder　pressure　vessel,　a　water　pressure　loading　system　to　simulate　the　actual　working　loading　condition,　and　strain　gauge　sets　for　strain　measurement.　A　number　of　experiments　have　been　done　by　cyclically　loading　and　unloading　the　vessel　at　different　strain　levels　of　plastic　deformation　to　shakedown　state　in　different　experiments　respectively.　The　relationship　between　plastic　strain　and　shakedown　range　has　therefore　been　obtained.　The　true　stress-strain　constitutive　model　of　the　vessel　under　large　deformation　condition　has　also　been　derived.　The　experiments　lead　to　three　observations:　(1)　the　strain　hardening　technology　can　effectively　reduce　the　residual　stress　of　stainless　steel　pressure　vessel.　(2)　The　shakedown　range　is　less　than　2000　mu　epsilon　for　a　total　strain　under　4%,　and　yet　the　shakedown　range　increases　fast　to　more　than　3000　mu　epsilon　for　a　total　strain　higher　than　6%.　(3)　It　is　also　found　that　the　true　stress-strain　constitutive　model　given　in　this　paper　describes　the　multi-axial　stress-strain　state　of　pressure　vessels,　and　validates　the　engineering　practicability　of　conventional　uniaxial　tensile　shakedown　constitutive　curve.