The　fatigue　crack　growth　rate　of　4130X　steel　used　in　compressed　natural　gas　vessel　in　H2S　environment　was　investigated　in　this　paper.　Considering　working　environment　and　stress　status　of　the　vessel,　modified　WOL　specimens　were　used　in　experiments,　the　frequency　of　experiments　was　0.0067Hz,　and　the　environments　of　experiments　were　saturated　H2S　environment,　moderate　concentration　and　laboratory　air.　The　relations　between　FCGR-da/dN　and　the　increment　of　fracture　toughness　Delta　K　were　determined　through　data　fit　based　on　Paris　method.　A　correlation　study　between　the　microscopic　features　of　the　crack　growth＇s　three　stages　and　the　tendency　of　stress　intensity　factor　(K)　to　the　test　samples　has　been　carried　out.　Then　the　quantitative　results　of　K　of　the　three　stages　in　different　environments　were　concluded.　The　results　show　that:　the　da/dN　in　H2S　environment　is　20　times　more　than　that　in　air　environment;　but　the　value　of　da/dN　doesn＇t　increase　when　the　H2S　density　reaches　to　a　certain　limit,　the　H2S　corrosion　medium　accelerates　the　fatigue　failure.