Abstract: Based on the thermal compression experiment on Gleeble-1500 thermal simulation machine, the constitutive equation of as-cast 42CrMo steel during hot compression deformation was derived through linear regression analysis for the test dadas. The effects of hot compression deformation parameters on microstructure were discussed. The results show that at the same deformation temperature (850-1 150 ℃), the grains size of the steel after deformation decreased with the increase of the strain rate, and it reached the minimum at the strain rate of 5 s-1. At the same strain rate (0.1-5 s-1), the microstructure decreased gradually, then increased with the increase of the deformation temperature, and the martensite lath reached the minimum at 1 050 ℃. The grains size decreased sighnificantly with the increase of the deformation at the same strain rate (1-5 s-1) and deformation temperature (900-1 050 ℃). The best refining effect could be obtained after recrystallization could be obtained at temperature of 1 050 ℃, strain rate of 5 s-1 and strain of 0.6.