Abstract: To investigate the defect formation mechanism and mechanical properties of aluminum matrix silicon carbide composites fabricated by laser additive manufacturing, 10 wt.% SiC particle-reinforced AlSi10Mg composites were prepared using selective laser melting. The effects of laser power and scanning speed on relative density, microstructure, and mechanical properties were analyzed. The results show that lack-of-fusion pores are the dominant defects, and their size and number first decrease and then increase with the energy input. Their formation is mainly attributed to the high melting point and hard characteristics of SiC particles, as well as the abrupt changes in melt rheological properties induced by them: On the one hand, SiC hinders molten pool flow, inducing lack-of-fusion particularly in particle-agglomerated regions; on the other hand, its high thermal conductivity intensifies local cooling and suppresses the complete melting of the matrix. In addition, laser energy input regulates the extent of interfacial reactions between SiC and the Al melt, affecting the formation of the Al₄SiC₄ reinforcing phase and the interfacial bonding state, which in turn influences defect behavior. Under optimal parameters, the material achieves the best comprehensive mechanical properties, with a tensile strength of 334.3 MPa and an elongation after fracture of 3.9%; whereas excessive defects lead to a performance deterioration of nearly 20%.