Numerical simulations can be truly reliable only when a proper material flow stress relationship is built because of its effective role on metal flow pattern as well as the kinetics of metallurgical transformation. Based on the hot compressive deformation behaviors of 42CrMo steel, a comprehensive constitutive model has been developed to predict stress–strain curve up to the peak stress. A new material parameter L, which is sensitive to the forming temperature and strain rate, was proposed in the developed constitutive model. Additionally, the mathematical models to predict peak stress and peak strain were established based on experimental results. The stress-strain values predicted by the developed model well agree with experimental results, which confirmed that the developed constitutive equation gives an accurate and precise estimate for the flow stress of 42CrMo steel.