The　calculation　of　a　temperature　field　has　a　great　influence　upon　the　analysis　of　thermal　stresses　and　stains　during　quenching.　In　this　paper,　a　42CrMo　steel　cylinder　was　used　an　example　for　investigation.　From　the　TTT　diagram　of　the　42CrMo　steel,　the　CCT　diagram　was　simulated　by　mathematical　transformation,　and　the　volume　fraction　of　phase　constituents　was　calculated.　The　thermal　physical　properties　were　treated　as　functions　of　temperature　and　the　volume　fraction　of　phase　constituents.　The　rational　approximation　was　applied　to　the　finite　element　method.　The　temperature　field　with　phase　transformation　and　non-linear　surface　heat-transfer　coefficients　was　calculated　using　this　technique,　which　can　effectively　avoid　oscillationin　the　numerical　solution　for　a　small　time　step.　The　experimental　results　of　the　temperature　field　calculation　coincide　with　the　numerical　solutions.