The in situ reaction equation of B4C and TiO2 was identified using thermodynamic calculations and XRD analysis. The optimum presintering process was determined according to investigating the effect of presintering temperature on the flexural strength and porosity of the porous B4C-TiB2 preform. The effect of in situ synthesized TiB2 on the reaction products and initial reaction temperature of B4C and Al was discussed based on DSC and XRD analysis. The results showed that the in situ synthesized TiB2 could effectively improve the mechanical properties of the B4C-TiB2-Al composite, elevate the initial reaction temperature of B4C and Al, change the reaction products, and moderate the reaction of B4C and Al. The mechanism of reaction between B4C and Al was discussed.