Abstract: The hot deformation behaviors of China low activation martensitic (CLAM) steel were investigated by the thermal simulation compression test on Gleeble-3500 thermal simulator under the temperature of 950 ℃ to 1 150 ℃, strain rate of 0.01 s^-1 to 5 s^-1 and true strain of 1.2. The constitutive equation and hot processing map of CLAM steel were established. And the microstructures of CLAM steel under the deformation condition of stable domains and instability domains were observed by Leica-DMI5000M metallographic microscope. The results show that the dynamic recrystallization exists at higher deformation temperatures and lower strain rates condition. And dynamic recovery mainly occurs at lower temperatures and higher strain rates. Specially, the instability domain at lower temperatures and higher strain rates should be avoided. By comparing and analyzing the hot processing map and microstructure, the optimum hot working domain of CLAM steel with the temperature of 955 ℃ to 1035 ℃ and the strain rate of 0.03 s^-1 to 0.32 s^-1 was obtained, and under the conditions the power dissipation efficiency is above 36%.