In the production process of epoxy/Al₂O₃ composite material, due to incomplete ring-closure reaction of epoxy resin matrix or incomplete washing with water, a certain amount of chlorohydrin residues remain in the by-products generated, resulting in the formation of hydrolyzable chlorine. As an impurity in epoxy resin matrix, hydrolyzable chlorine will affect the electrical properties of epoxy resin composites. In order to explore the influence rule and action mechanism of hydrolyzable chlorine on the electrical properties of epoxy/Al₂O₃ composites, four kinds of epoxy resins with different hydrolyzable chlorine content were obtained by adding different quality of chlorohydrin ether into epoxy resin. And the electrical properties of epoxy/Al₂O₃ composite material samples were tested, including breakdown strength, dielectric constant, dielectric loss factor, and volume resistivity. By analyzing the electrical properties of samples, the effects of different hydrolyzable chlorine content on the electrical properties of epoxy/Al₂O₃ composites were attributed to the strong electronegativity of Cl atoms in hydrolyzable chlorine. The experimental results show that with the increase of hydrolyzable chlorine content, the breakdown strength of epoxy/Al₂O₃ composites increases, the volume resistivity decreases, and the dielectric constant and dielectric loss factor both decrease slightly.