Abstract: The plastic deformation behavior of magnesium alloy(AZ91D) and chopped carbon fiber(CFs)/AZ91D composites with different fiber volume fractions were investigated at the condition of 310-430℃ and 10^-3-10^-1 s^-1 using isothermal compression testing. According to the experimental results, the hot processing maps of AZ91D and CFs/AZ91D were constructed to inspect the influence of fiber on the plastic workability and deformation mechanism of CFs/AZ91D composites. The results indicate that the addition of the fiber improves the flow stress as well as the dynamic recrystallization (DRX) and strain softening degree. Nevertheless, the fiber volume fraction has no significant impact on the flow stress and the strain softening degree. The strain rate sensitivity exponent and deformation activation energy of CFs/AZ91D composites are higher than that of AZ91D. It is found there is no deformation instability zone in the processing map of the AZ91D and the power dissipation efficiency exceeds 30% in the deformation safety zone. Due to the stimulation of fiber on the DRX in matrix alloy, the power dissipation efficiency of CFs/AZ91D composite is ultra-high, which exceeds 50%. At the condition of low temperature and high strain rate, the large deformation is liable to induce the interface debonding, which is responsible for the plastic deformation instability of CFs/AZ91D composites.