The invention discloses a size graded furnace slag granulating device, and belongs to the technical field of blast furnace slag utilization. The size graded furnace slag granulating device comprises aslag loading unit, a granulating unit, a screening unit and a housing, wherein the granulating unit is arranged in the housing, and comprises a rotating slag pan and a cooling mechanism; a heating component is arranged on the rotating slag pan; a slag particle shower nozzle, a liquifying shower nozzle and an atomizing shower nozzle of the cooling mechanism are arranged on the peripheral edge of the rotating slag pan; the slag loading unit comprises a slag pot and a slag outlet channel; the slag outlet channel is formed in the bottom of the slag pot; and an outlet of the slag outlet channel isformed above the rotating slag pan. A slag film is formed on the edge of the rotating slag pan, and blast furnace slag on the edge of the rotating slag pan is heated and thermal-insulated by the heating component, so that the slag film can be maintained at a higher temperature, the slag film formation is inhibited, and the reduction of the particle size of granulated slag is facilitated; the screening unit is positioned at the discharging opening below the housing, and is used for screening the granulated slag to obtain size graded granulated slag, and the resource utilization of the blast furnace slag is realized.