The invention discloses a special composite liquid grinding aid for blast furnace slag. The special composite liquid grinding aid comprises components in parts by weight as follows: 1-10 parts of triisopropanolamine, 1-10 parts of triethanolamine, 10-20 parts of mixed alcohol, 1-10 parts of magnesium aluminum silicate, 1-10 parts of ethyl acetate, 5-20 parts of sugarcane molasses, 1-10 parts of sodium carbonate and the like. A preparation method of the special composite liquid grinding aid for the blast furnace slag comprises steps as follows: A, in terms of parts by weight, water is injected into a reactor and heated, then sodium hexametaphosphate, anhydrous sodium metasilicate and sodium acetate are added, and a solution I is obtained; B, sodium carbonate and magnesium aluminum silicate are added to the solution I, and a solution II is obtained; C, triisopropanolamine, triethanolamine, mixed alohol, ethyl acetate and the sugarcane molasses are added to the solution II and stirred, then sugarcane molasses alcohol liquid waste and a polycarboxylate superplasticizer are added, a mixed solution is stirred again and sieved, and a filtrate obtained after filtering is taken as the special composite liquid grinding aid for the blast furnace slag. The grinding aid can reduce the powder surface free energy, can be universally applicable, adopts a simple and understandable preparation technology and has great popularization value.