Abstract: Gleeble thermal simulation testing machine was used to carry out the thermal compression experiments with deformation amount of 60%. The true stress-true strain curves at high temperature of 950-1200℃ and strain rate of 0.001-1 s^-1were obtained. The results show that the flow stress curves show the dynamic recrystallization characteristics under the most deformation conditions. The rheological stress increases with the deformation temperature decreasing and the strain rate increasing. The hot deformation activation energy is calculated to be 382.5122 kJ/mol. A "two-stage" high-temperature flow stress constitutive model was constructed. The correlation coefficient between the calculated and experimental values of the model is as high as 0.99784. The critical and peak strains were determined by obtaining the work-hardening rate using the graphical method. A dynamic recrystallization model of 42CrMo4 steel considering the hot deformation characterization was established. The high temperature constitutive model and the microstructure evolution model were embedded into the Deform 3D subroutine to perform a numerical simulation of upsetting of a workpiece to obtain the average grain size and the dynamic recrystallization volume fraction. The results show that the simulation results are in good agreement with the experimental results, which verifies the accuracy of the constitutive model of 42CrMo4 steel.