Abstract: Large strain isothermal compression tests were carried out on a Gleeble-3800 thermal simulator to study the high temperature deformation behavior of Cr-Co-Mo-Ni gear steel. The constitutive equation and hot processing map of the steel were established based on experiment data. The results show that the flow stress decreases with increasing temperature,but increases with increasing strain rate. The flow stress can be described by the constitutive equation in a hyperbolic sine function and the averaged value of activation energy is 487.21 kJ·mol^-1. The proper hot working regions shown by the hot processing map are the temperature of 1000-1100℃ and the strain rate of 0.1-1 s^-1. In addition,the forging processes were simulated by finite element method based on the experiment,the effects of initial forging temperature and passes on the temperature and strain rate of forgings were analyzed,and the proper forging process parameters were obtained which are the initial forging temperature of 1000-1100℃ and the forging passes of 15.