The invention relates to a medical X-ray-proof composite material and a preparation method thereof and belongs to the technical field of X ray protection. According to the medical X-ray-proof composite material, a nano lanthanum hexaboride (LaB6) and high-temperature-resisting liquid crystal polyester composite material is used as a shielding material and the problem of dangers of heavy metal leadis solved; a novel high molecular composite material which has the advantages of simple preparation method and easiness for bending and molding is obtained; meanwhile, the organically-treated nano lanthanum hexaboride modified high-temperature-resisting liquid crystal polyester composite material is added into graphene modified copolymerized polyester and procedures including melt stretching arecarried out to prepare a thickness-controllable composite sheet material; the composite sheet material is subjected to multilayered hot pressing through a hot pressing machine to obtain a medical X-ray-proof composite board with good comprehensive shielding performance; the material is light and convenient to operate.