Y3Al5O12-Al4SiC4 composite refractory was prepared via pressureless sintering with Al4SiC4 as a raw material and Y2O3 as an additive. The effect of Y2O3 addition (i.e., 0-5% in mass fraction) on the phase composition, microstructure and mechanical properties of the composite was investigated, and the effect of Y3+ ions on the surface energy of Al4SiC4 different crystal faces was calculated via the DFT simulation. The results show that the addition of Y2O3 can form an eutectic liquid phase with anintrinsic oxide layer on the surface of Al4SiC4 at a high temperature, Y3Al5O12 and a small amount of Y2SiO5 appear at the grain boundary of the composite.The addition of Y2O3 promotes the growth of Al4SiC4 grains along the two-dimensional plane and the transformation from irregular to hexagonal flakes due to the reduction of surface energy of different crystal faces of Al4SiC4 by Y3+ in the additive, according to the DFT results. The flexural strength of the composite first increases and then decreases with the increase of Y2O3 addition. At theY2O3 addition of 3%, the hexagonal lamellar Al4SiC4 grains with a uniform size form a staggered skeleton structure, and Y3Al5O12 dispersesat the grain boundary of Al4SiC4, which is conducive to the improvement of the mechanical properties of Y3Al5O12-Al4SiC4 composite. The bulk density, apparent porosity and flexural strength of the composite are 2.44 g·cm-3, 20.15% and 52.7 MPa, respectively.