The unidirectional compression deformation of SiC_p/AZ91 magnesium matrix composites was carried out by Gleeble-3800 thermal simulation testing machine under the deformation temperature of 250 ℃～400 ℃ and the strain rate of 0.001 s^-1～1 s^-1. The microstructure evolution during the deformation process was observed and the high temperature deformation mechanism of SiC_p/AZ91 magnesium matrix composites was revealed. The results indicate that there are only Mg phase and SiC phase in as-cast SiC_p/AZ91 magnesium matrix composites. When the strain rate is 0.001 s^-1, composites are partially recrystallized at 250 ℃, which are fully recrystallized at 350 ℃. When the compression temperature is increased to 400 ℃, the recrystallized grains grow rapidly. The reduction of strain rate is beneficial to dynamic recrystallization of SiC_p/AZ91 magnesium matrix composites at 250 ℃ and 350 ℃, where the stress factor is 5, the true activation energy Q is 91 kJ/mol, and the deformation mechanism is dislocation climbing controlled by grain boundary diffusion.