(100-x) at.% Ba0.6Sr0.4TiO3-x at.% Y2O3 (BST/Y2O3) composite ceramics were prepared by conventional solid-state reaction methods. Phase constitution, microstructure, and element distribution of the composite ceramics were characterized, and the dielectric properties of BST/Y2O3 composite ceramics were measured. The results revealed that some of Y3+ were dissolved into the lattice of BST and the rest remained at grain boundaries in oxide form of Y2O3. With the increase of the content of Y2O3, Curie peak of BST/Y2O3 composite ceramics moved to a lower temperature, the dielectric permittivity and dielectric loss of BST/Y2O3 composite ceramics decreased significantly, meanwhile, the dielectric tunability still maintains a relatively high value. Furthermore, a multipolarization mechanism model is employed to describe the field dependence of the dielectric permittivity in BST/Y2O3 composite, which suggests that the existence of the second phase of Y2O3 diminishes the contribution of polar clusters in BST matrix, leading to the decrease of dielectric tunability. P-E loop measurement indicates that the breakdown strength of BST/Y2O3 ceramics increases during addition of Y2O3 in BST matrix. Therefore, the BST/Y2O3 composites with good tunable dielectric properties and enhanced breakdown strength can be exploited for phase shifter and capacitor device applications.