Fatigue　crack　growth　rate　da/dN　of　the　material　4130X　steel,　which　is　used　to　manufacture　the　high-pressure　vessel,　in　moderate　concentration　(200　x　10(-6))　sulfureted　hydrogen　solution　is　studied　experimentally.　The　equation　of　fatigue　crack　growth　rate　is　found　according　to　the　experimental　data　by　using　Paris　expression.　Fatigue　crack　growth　rate　equation　of　90%　reliability　and　90%　believability　is　also　found　taking　into　account　experimental　randomicity.　By　using　inverse　inferring　method,　allowable　crack　sizes　with　variable　crack　shape　are　determined　in　certain　setting　inspection　period　to　assure　the　safety　of　the　vessel.　And　the　relationship　between　allowable　crack　depth　and　original　crack　length　is　obtained　and　shown　with　figures　which　are　very　convenient　for　engineers　in　evaluating　residual　life　of　the　pressure　vessel.