Abstract: Based on the research background of casting-forging combined forming process, as-cast 40CrNiMo alloy steel was obtained by vacuum casting by using metal mold. The isothermal compression tests of as-cast 40CrNiMo are implemented on a Gleeble-3800 thermal simulated test machine under deformation temperature of 800-1100 ℃, with strain rate of 0.001-10 s^-1.The true stress-strain curves of the as-cast 40CrNiMo alloy steel were plotted according to the experimental data. The results show that the true stress-true strain curves present typical dynamic recovery type under low deformation temperature or high strain rate; with the deformation temperature increasing or the strain rate decreasing, the true stress-strain curves gradually transform to the dynamic recrystallization type. Johnson-Cook constitutive model of as-cast 40CrNiMo alloy steel is constructed and the strain rate sensitivity parameter is modified. The results show that the model has good applicability at low temperature and high strain rate. So the Johnson-Cook constitutive model of as-cast 40CrNiMo alloy steel is constructed under applicable conditions and compared with the Arrhenius constitutive model. On the whole, the prediction accuracy of the Johnson-Cook constitutive model is better than that of the Arrhenius constitutive model, but under the condition of deformation temperature 900 ℃ and strain rate 10 s^-1, the prediction accuracy of the Arrhenius constitutive model is slightly better than that of the Johnson-Cook constitutive model.