Near-net-shape SiC_p/A356 composites baseplate was successfully prepared with SiC_p preform by pressureless infiltration method. Bonding mechanism of sintered SiC preform, and effects of infiltration temperature and infiltration time on the relative density of the composites were approached. Meanwhile, properties of the composites baseplate were tested. The results reveal that SiO2 phase generated on the surface layer of sintered SiC is responsible for the bonding role in SiC porous preform. Relative density of the composites baseplate is increased with increasing in infiltration temperature and infiltration time. Thermal expansion coefficient and thermal conductivity of the composites baseplate vary at （11.15¹2.46）×10-6 K-1 and at 110.001⁹4.282W/（m·K）, respectively, at 50²00℃. The ambient bending strength of the composites baseplate reaches 210.8MPa.