Using the self-developed mechanical stirring machine for preparing continuous semi-solid slurry of composites，semi-solid slurry of composites B4C/Y112 was prepared. The effects of some parameters including preheating treatment of B4C particles, different size of B4C particles and different mass fraction of B4C particles on the micro structural evolution in the slurry were described. Then the tensile, hardness and other mechanical properties of materials prepared with different mass fraction of B4C particles（particle size 57μm）were tested. The results show that after preheating treatment of B4C particles, increasing in size and mass fraction of B4C particles, grain in the slurry becomes fine, uniform and round, but the solid fraction reduces. Under the condition of appropriate parameters: 400 ℃ preheating treatment for 2 hours, 57μm size and 6% mass fraction of B4C particles, grain in the slurry becomes fine uniform and round. The hardness of semi\|solid aluminum matrix composites increased with the increasing of concentration of B4C particles, 12wt.%B4C/Y112 composite had the highest hardness, reached to 136HV. With increasing of the mass fraction of B4C particles, its tensile strength increased first and then decreased.