Abstract: The hot deformation behavior of Fe-0.15 C-7 Mn medium-Mn steel was investigated by single-pass compression experiments in the Geeble-3500 thermal simulator at deformation temperatures of 900 ℃-1150 ℃ and strain rates of 0.001 s^-1-1.0 s^-1. The flow stress-strain curves of the steel were obtained. Strain compensation was introduced into the constitutive equation based on the assumption that the material constant was a polynomial function of strain. The strain-compensated constitutive equation of the steel was established by fitting the experimental data and introducing the Zener-Hollomon parameter. The thermal deformation activation energy of the steel is 321615 J/mol. In addition, the model of critical/peak strain and grain size evolution of the steel was established. The simulation results of constitutive equation are consistent with the experimental results. The correlation coefficient（R） and average absolute relative error（AARE） are 0.96%and 7.38%, indicating the reliability of the model.