The　fracture　toughness　of　No.　45　steel　before　and　after　corrosion　is　studied　in　this　paper.　Two　experiments　have　been　carried　out　about　hardness　experiment　and　COD　experiment　before　and　after　corrosion.　And　the　influence　of　corrosion　condition　of　the　hardness　increment　and　fracture　toughness　is　analyzed　here.　From　the　hardness　test,　we　find　that　the　hardness　of　No.　45　steel　rise　at　first　and　then　fall　down　to　the　original　hardness,　and　the　maximum　increment　of　No.　45　steel　is　21.　From　the　COD　experiment,　we　find　that　the　stress　intensity　factor　of　No.　45　steel　before　and　after　corrosion　are　190.7Mpam(1/2)　and　149.8　Mpam(1/2)　respectively　while　their　corresponding　average　stress　of　crack　tip　are　1076Mpa　and　845.6Mpa.　It　can　be　seen　that　hydrogen　sulfide　corrosion　increases　the　brittle　facture　sensitivity　of　45　steel,　reduces　the　fracture　toughness,　and　produces　crack　tip　stress　of　230.5Mpa.　When　we　take　the　hardness　of　the　experimental　results　into　the　hardness-stress　calibration　formula,　the　hydrogen　corrosion　generates　239Mpa　hydrogen　stress.　The　results　of　experiment　hardness　experiment　and　COD　experiment　are　less　than　10%　of　the　crack　stress　in　the　COD　experiment,　which　proves　that　hydrogen　corrosion　will　cause　hydrogen　stress　and　then　lead　to　the　decrease　of　fracture　toughness.