Owing to the complementary mechanical properties and good physicochemical compatibility of Ti3SiC2 and Al2O3, Ti3SiC2/Al2O3 composites are considered as ideal structural materials. However, the low purity of Ti3SiC2 phase is the bottleneck problem in the preparation of Ti3SiC2/Al2O3 composites through an in-situ reaction. In this study, Ti3SiC2/Al2O3 composites with different Al contents were prepared at different temperatures via hot pressing sintering. Al, as a sintering aid, improved the purity of the Ti3SiC2/Al2O3 composites. This is because Al could partially replace Si in Ti3SiC2 lattices, which stabilized the lattice of Ti3SiC2. Furthermore, the introduction of an appropriate amount of Al could promote the densification sintering of Ti3SiC2/Al2O3 composites and improve the mechanical properties. The sample prepared at 1450 °C with 0.2 mol Al addition showed the best comprehensive properties, whose relative density, Vickers hardness, flexural strength, and fracture toughness were 99.48%, 16.72 GPa, 608.21 MPa, and 8.73 MPa.m1/2, respectively.