The thermal compression experiment of Fe-27.51Mn-8.69Al-1.12C low density steel under the temperature of 900-1100 ℃ and the strain rate of 0.01-5 s-1 was carried out by thermal simulation experiment machine Gleeble-1500D, and the rheological stress curves characteristics of the steel were analyzed by the experimental data. Then, the physical constitutive model considering strain coupling was established, and the verification analysis was conducted. The results show that the thermodynamic conditions such as deformation temperature and strain rate have significant effects on the rheological stress of low density steel, and the high temperature and low strain rate are more conducive to the recrystallization of low density steel. The prediction accuracy of physical constitutive equation for low density steel under isothermal compression based on peak stress is high, and the linear fitting correlation coefficient is 0.991. The physical constitutive equation of low density steel under isothermal compression based on strain compensation can better describe the change rule of rheological stress for low density steel under hot compression, the correlation coefficient r is 0.980, and the average relative error AARE between the predicted and experimental values is 6.9%.