In the present work, in order to obtain the magnesium matrix composites with good mechanical properties, the semi-solid billet 10μm 15wt.%SiC/AZ91D was prepared using the near-liquidus heat-holding method and later semi-solid extruded to get the magnesium matrix composites. Next, 24h of solution treatment at 415oC (T4) was conducted, followed by 8h of further aging treatment at 220oC (T6). The results show that, with the progress of solution and aging treatments, Mg17Al12 phases at the grain boundary were dissolved, and secondary-precipitated in grains with layered and spheroidized gradually. The magnesium matrix composites 15wt.% SiC/AZ91D after T6 treatment showed better integrated mechanical properties when compared with those before, with tensile strength, yield strength, elongation, and hardness reaching 242MPa, 204MPa, 2.3% and 132.26HV, respectively.