The invention discloses a compound slag former in an LF furnace. The compound slag former is prepared from 70 to 100 parts of aluminum cell waste cathode carbon blocks and 0 to 30 parts of aluminum ash, the aluminum cell waste cathode carbon blocks and the aluminum ash are smashed to below 5 mm and are mixed for 10 to 20 min, and the mixture is dried at the temperature of below 300 DEG C, so that the water yield of the mixture is smaller than 0.5% of the total weight of the mixture to obtain the compound slag former in the LF furnace. A deoxidized slag agent is prepared by using the aluminum cell waste cathode carbon blocks and the aluminum ash, aluminum contained in the aluminum ash and carbon contained in the aluminum cell waste cathode carbon blocks are added into the LF furnace to emit a great deal of heat, the heat loss in the process that the compound slag former is added into the LF furnace is recovered, and the waste cathode carbon blocks can be used for recarburization for molten steel; the aluminum cell waste cathode carbon blocks and the aluminum ash are cheap, so that the production cost of the compound slag former in the LF furnace is reduced.