In-situ TiB2/AlSi10Mg composites were prepared by casting, gas atomization, and selective laser melting (SLM), respectively. Microstructural evolution of the composites in as-cast, atomized, and SLMed were investigated. Compared with the as-cast composite, the SLMed sample has no texture and is composed of extremely fine grains and crystal cells. The in-situ TiB2 particles uniformly distribute over the grain boundaries, the acicular Si phase is replaced by a network of fine Si particles and there are nano-sized Si precipitations in the crystal cell. The formation of this microstructure is due to the continuous solidification of non-equilibrium eutectic Al–Si during the rapid cooling process of SLM. The SLMed TiB2/AlSi10Mg composite obtains excellent strength (tensile strength ∼ 451 MPa, bending strength ∼ 674 MPa), hardness (∼151 HV0.1) and good friction coefficient (∼0.4), which are higher than that of the as-cast TiB2/AlSi10Mg composite and the SLMed AlSi10Mg alloy. The formation mechanism of this particular structure and its strengthening effect were discussed.