The 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. The results demonstrated that the flow stress curves show typical dynamic recrystallization (DRX) characteristics. It was discovered by EBSD that the nucleation and growth of DRX grains are mainly at grain boundaries. The complete DRX occurred at 1100℃/0.01s-1, and the grain was obviously refined. The main DRX nucleation mechanism of the studied alloy is the grain boundary bowing nucleation. Therefore, the softening mechanism of Fe-Cr-Ni-based alloys for nuclear power plants was a combination of dynamic recovery (DRV) and DRX. The Arrhenius constitutive model with strain compensation was developed. The correlation coefficient between the predicted and experimental values is 0.9947. The mathematical model of critical stress (strain) and Z parameter were reliably obtained. The critical stress (strain) of DRX increases as temperature decreases and strain rate increases. The DRX kinetic model was established by the Avrami model, and a typical S-type curve was obtained. As the strain rate decreases and the temperature increases, the volume fraction of DRX will increase.