Porous preforms were fabricated by cold-pressing process using powder mixture of TiC, TiO_2 and dextrin. After pyrolysis and sintering, Al melt was infiltrated into the porous preforms, leading to the formation of Ti_3AlC_2-Al_2O_3-TiAl_3 composite. Effects of cold-pressing pressure of preforms on microstructures and mechanical properties of the composites were studied. Synthesis mechanism and toughening mechanism of composite were also analyzed. The results shows that TiO_2 is reduced into Ti_2O_3 by carbon, the decomposition product of dextrin, which causes the spontaneous infiltration of Al melt into TiC/Ti_2O_3 preform. Then, Ti_3AlC_2-Al_2O_3-TiAl_3 composite is in-situ formed from the simultaneous reaction of Al melt with TiC and Ti_2O_3. With the increase of cold-pressing pressure from 10 MPa to 40 MPa, the pore size distribution of the preforms becomes increasingly uniform after pre-sintering, which results in the reduction of defects, and the decrease of property discrepancy of composites. Nano-laminated Ti_3AlC_2 grains and Al_2O_3 particles make the fracture toughness of TiAl_3 increase remarkably by various toughening mechanisms including stress-induced microcrack, crack deflection and crack bridging.