The　threshold　in　fatigue　crack　propagation　(ΔKth),　which　is　a　principal　indicator　of　the　property　resistant　to　fracture　of　members　bearing　crack　defect　during　the　process　of　corrosion　fatigue,　plays　an　important　role　in　engineering　project.　ΔKth　cannot　be　directly　obtained　from　test　methods　due　to　the　accurate　measurement　of　crack　extension　could　not　be　conducted　with　the　current　test　methods.　The　over-and-under　method　of　fatigue　reliability　theory　is　innovatively　introduced　into　the　test　determination　of　ΔKth.　Meanwhile,　a　flatbase　for　the　determination　of　ΔKth　in　low　cycle　fatigue　H2S　environment　is　set　up.　Additionally,　data　analysis　is　conducted　by　the　way　corresponding　to　the　over-and-under　method.　The　results　show　that:　1)The　test　determination　of　ΔKth　can　be　effectively　obtained　through　the　test　flatbase　of　corrosion　fatigue.　2)The　accurate　measurement　and　study　of　ΔKth　in　low　cycle　fatigue　H2S　environment　is　realized　by　the　over-and-under　method　of　fatigue　reliability　theory.　3)The　ΔKth　in　air　environment　is　obviously　greater　that　the　one　in　low　cycle　fatigue　H2S　environment,　where　they　differ　by　more　than　50%.　©　2009　SPIE.