The invention discloses a special composite slag removing agent for beryllium aluminum alloy refining, a preparation method and a slag removing method. The slag removing agent comprises the followingcomponents in parts by weight: 30-50 parts of sodium tetraborate, 2-8 parts of AlB12 intermediate alloy, 2-5 parts of hexachloroethane, 7-10 parts of sodium hexafluorisilicate, 2-6 parts of sodium fluoroaluminate, 1-5 parts of sodium fluoride, 5-12 parts of potassium chloride, 6-13 parts of beryllium chloride, 2-5 parts of lithium chloride, 2-5 parts of sodium carbonate, and 1-3 parts of clay. Theslag removing agent achieves excellent high-temperature stability and reliability under the vacuum condition, and is excellent in adhesiveness after melting, high in spreadability, quick in slag collection, easy incrustation of slag skin and excellent in slag removing and refining effect. Through the synergistic effect of all the components, oxides in melts, magnesium-contained impurities and hydrogen achieve excellent slag removing and absorbing effects; and meanwhile, the slag skin can effectively cover the surfaces of the melts, so that the temperature loss can be effectively reduced, thesplashing of the melts is inhibited, and new melting slag and secondary oxidation are prevented.