Abstract: The 3wt% TiB₂/Al-4.5Cu composites with different mass fraction of Mg were fabricated by Al-K₂TiF₆-KBF₄ molten salt reaction method. The influences of Mg and multistage heat treatment on the microstructure and mechanical properties of the 3wt% TiB₂/Al-4.5Cu composite were studied by means of SEM, TEM, HM hardness test and room temperature tensile test. The microstructure observation shows that the agglomeration of TiB₂ particles is obviously improved after multistage heat treatment along with addition of Mg, In particular, with 3wt% Mg, the average size of TiB₂ particles is about 130 nm, accompanied by a large number of dispersed nano particles, and the grain size of α-Al is also significantly reduced. The mechanical test results show that after multistage heat treatment, the hardness and tensile strength of the 3wt% TiB₂/Al-4.5Cu composite are enhanced with the increase of Mg content. However, excess Mg content (mass fraction≥4wt%) will decrease the refinement effect of TiB₂ particles. The analysis reveals that addition of Mg can reduce the interfacial energy of TiB₂/α-Al and block the formation of brittle phase Al₃Ti, Al₂B. Meanwhile, the MgAl₂O₄ formed by reaction makes the interface become into the TiB₂/MgAl₂O₄/α-Al, which could restrain the agglomeration of TiB₂ particles and improve the wettability of the interface between TiB₂ particles and Al liquid, increase the nucleation rate and refine the grain size.