Thermal deformation characteristics of Fe-Cr-Ni-based alloys for nuclear power plants were investigated using a Gleeble-3500 thermal simulation tester. The microstructure evolution law of alloy heat deformation was investigated using the electron backscatter diffraction (EBSD) technique. Results demonstrate that the flow stress curves show typical dynamic recrystallization (DRX) characteristics. According to EBSD analysis, the nucleation and growth of DRX grains are mainly at grain boundaries. The complete DRX occurs at 1100 °C/0.01 s^-1 condition, and the grains are refined. The main DRX nucleation mechanism of the alloy is the grain boundary bowing nucleation. Therefore, the softening mechanism of Fe-Cr-Ni-based alloys for nuclear power plants is the combination of dynamic recovery and DRX. The Arrhenius constitutive model with strain compensation is developed. The correlation coefficient between the predicted and experimental values is 0.9947. The reliable mathematical model of critical stress (strain) and Z parameter is obtained. The critical stress (strain) of DRX increases as the temperature decreases or the strain rate increases. The DRX kinetic model is established by the Avrami model, and a typical S-type curve is obtained. As the strain rate decreases and the temperature increases, the volume fraction of DRX increases.