This　paper　studies　the　spontaneous　buckling　morphology　transition　of　an　elastic　ring　confined　in　an　annular　region　constraint.　With　the　increase　of　uniform　axial　strain,　the　ring　initially　forms　a　symmetrical　blister,　and　then　the　blister　is　compressed,　spontaneously　inclines　and　transits　to　the　＂S＂　shape.　A　theoretical　framework　based　on　the　minimum　potential　energy　theory　is　proposed　to　obtain　the　critical　strain　of　morphology　transition,　which　matches　well　with　experimental　results.　The　map　predicting　stable　buckling　shapes　of　the　ring　with　different　geometrical　parameters　is　demonstrated,　which　may　offer　helpful　guidance　to　practical　applications,　for　example,　the　design　of　gear-less　pump　and　rotary　actuators.