For 12%Cr heat-resistant steel, the high-temperature tensile experiments were conducted by thermal simulation test machine Gleeble-3800D, and the high-temperature tensile true stress-true strain curves at different deformation temperatures and strain rates were obtained. The results show that the higher the strain rate and the lower the temperature, the greater the tensile strength and yield strength of  material. The microstructure of high-temperature tensile fracture indicates that the higher the temperature and the lower the strain rate, the better the plasticity of material. By combining the high-temperature tensile experiments and numerical simulation methods, the critical damage value during the hot forging process of 12%Cr heat-resistant steel was determined, and the critical damage value for crack initiation was further converted into the actual critical deformation amount during the hot forging process. A critical deormation amount model for the hot forging process of 12%Cr heat-resistant steel under the given deformation temperature and strain rate conditions was established, and the upsetting process was verified by 500 N hydraulic press, which provides the relevant theoretical references for the forging process of ultra-supercritical rotors.