Abstract: Based on the true stress-true strain curves of 45^# steel under three kinds of heat-treated conditions obtained at strain rates from 10^-3s^-1~10³s^-1 by means of material test system(MTS) and split Hopkinson pressure bar(SHPB), the undetermined coefficients of the Johnson-Cook constitutive relations were fitted with least squares theory, and the strain rate hardenability value was corrected. It is proved that the constitutive relation parameters are in agreement with the experimental results. The contrast between numerical simulation and experimental data obtained by SHPB demonstrate that the corrected Johnson-Cook constitutive relation can describe the mechanical behavior of 45^# steel under impact loading. According to the analysis referred to above, the quantitative relation between the constitutive relation parameters and the content of ferrite is determined, which provides rules for selecting the heat treatment processes of 45^# steel at high loading rate.