Based　on　the　temperature　rise　caused　by　adiabatic　process　and　the　viscosity　of　the　metal　under　the　impact　of　steel,　the　Johnson-Cook　constitutive　relation　of　structural　steel　in　civil　engineering　was　modified.　Applying　the　Split-Hopkinson　Pressure　Bar　(SHPB)　testing　technique,　the　impact　experiment　under　elevated　temperatures　(25,　350,　450　and　750　degrees　C)　and　high　stain　rates　(2300　and　3200s(-1))　of　Q345　steel　was　performed.　Johnson-Cook　constitutive　model　of　structural　steel　was　modified　on　the　basis　of　adiabatic　warming　effect　and　Perzyna　constitutive　model.　Based　on　the　result　of　the　experiment,　the　fitting　curves　were　drawn　and　compared　with　the　revised　constitutive　model.　The　results　showed　that　Q345　steel　has　obvious　strain　rate　hardening　effect　and　temperature　soften　effect,　and　the　curves　of　the　modified　constitutive　model　and　the　experimental　results　matched　very　well.　The　modified　Johnson-Cook　constitutive　relationship　can　precisely　describe　the　dynamic　characteristic　of　Q345　steel　under　the　impact　of　temperature　effect　and　strain　rate　effect.