The purpose of this study is to find the hot deformation behavior of as-cast 42CrMo steel. The thermal simulation experiments of as-cast 42CrMo steel were done on the Gleeble-1500 thermo-mechanical simulation machine. The hot deformation behavior of as-cast 42CrMo steel was analyzed. The true stress-strain curves in hot deformation at different deformation temperature (850°C,950°C,1050°C,1150°C), different strain rate (0.05S-1, 0.5 S-1, 1 S-1,5 S-1) were obtained. The influence rules of the deformation temperature and strain rate on the curves were analyzed. The analysis shows that the true stress increases with increase of deformation temperature and decrease of stain rate. The hot deformation behavior of as-cast 42CrMo steel was compared with forged 42CrMo steel. The results show that the flow stress of as-cast 42CrMo steel during hot deformation was higher than that of the forged 42CrMo steel. Compared to the forged 42CrMo steel, the dynamic recrystallization in as-cast 42CrMo steel during hot deformation is more difficult to occur. The above conclusions have significant theoretical and practical meanings for the design of hot deformation process of as-cast 42CrMo steel.