Abstract: CaO-B₂O₃-SiO₂(CBS) glass powder and Al₂O₃ ceramic powder were used as the raw materials. Appropriate amount of Bi₂O₃, which was used as flux, was added into the glass-ceramic composites consisting of CBS and Al₂O₃ with a fixed mass ratio of 50:50. The effects of Bi₂O₃ flux on the sintering character, dielectric properties, bending strength and thermal expansion coefficient of CBS/Al₂O₃ composite were investigated. The results show that Bi₂O₃ flux can lower the transition temperature and the viscosity of CBS glass, which promotes the densifying of CBS/Al₂O₃ composites, and results in the CBS/Al₂O₃ composite which has relatively dense structure and less porosity can be prepared by sintering at 880 ℃. However, the excess addition of Bi₂O₃ will deteriorate the sintering character, dielectric properties and bending strength of CBS/Al₂O₃ composites for the viscosity of CBS glass is too low. When the additive amount of Bi₂O₃ is 1.5wt% of CBS/Al₂O₃ composite, the densest CBS/Al₂O₃ composite which has a density of 2.82 g·cm^-3can be obtained by sintering at 880 ℃. This CBS/Al₂O₃ composite also has excellent dielectric properties (dielectric constant is 7.21, dielectric loss is 1.06×10^-3), the bending strength is 190.34 MPa, and the thermal expansion coefficient from 0 to 300 ℃ is 3.52 ×10^-6 K^-1.