Critical strain is the basis to determine whether the dynamic recrystallization occurs during hot deformation process, it is important for designers to control hot forming process and microstructure of final product. In order to investigate the influence of deformation temperature and strain rate on critical strain of Cr4 steel, hot compression simulation experiment was conducted for Cr4 steel in the temperature range of 750～1200°C and strain rate of 0.002～5S-1 by means of Gleeble−1500D thermo-simulation machine. The results showed that the critical strain increase with decreasing deformation temperature and increasing strain rate. Based on the experimental data and regression analysis method, the mathematical model of the critical strain of Cr4 steel is constructed. The critical strain model provides data support to predict the dynamic recrystallization during Cr4 heavy supporting roller hot forging process.