Abstract: Thermal simulation compression experiment was carried out on 20CrNi2Mo steel by using phase change instrument, and the steel's thermal compressive deformation behavior at 840℃ under different deformations (0-50%) was studied. The steel's microstructure and hardness were also analyzed. The results show that the flow stress curve of 20CrNi2Mo steel exhibits obvious dynamic recrystallization characteristics. When the deformation amount reaches 10%, the dynamic recrystallization occurs. With the increase of the thermal compression deformation, the grain size is refined and subsequently can almost be kept stable. When the deformation amount reaches 50%, the specimen is completely recrystallized. In addition, with the increase of the thermal compression deformation, martensite is refined gradually, the content of residual austenite decreases. When the compression deformation changes from 10% to 20%, the hardness increases significantly, and then tends to stabilization.