NOVELTY - Production of composite fertilizer using phosphate ore tailings as filler includes drying and crushing phosphate ore tailings; reacting with 93-98 mass% sulfuric acid and 2-5 mass% acidic wastewater containing phosphorus pentoxide (P2O5) from phosphoric acid factory in a reactor; controlling the stirring speed; baking the obtained product; piling up at room temperature; crushing into powder composite fertilizer filler; mixing with urea, ammonium phosphate, potassium sulfate, or potassium chloride; adding with water; pelletizing; and drying.    USE - Production of composite fertilizer (claimed).    ADVANTAGE - The method uses phosphorus in phosphorus tailings and acidic wastewater of phosphorus acid factory to reduce the consumption of phosphorus raw material in producing composite fertilizer; reduces the cost of composite fertilizer; relieves the land pressure of piling the phosphorus tailings; alleviates the environmental harm brought by phosphorus tailings and wastewater; and reduces the maintenance costs of tailings pond and processing costs of acidic wastewater from phosphorus acid factory.    DETAILED DESCRIPTION - Production of composite fertilizer using phosphate ore tailings as filler comprises:    (A) drying and crushing phosphate ore tailings; reacting with 93-98 mass% sulfuric acid and 2-5 mass% acidic wastewater containing P2O5 from phosphoric acid factory in a reactor, where mass ratio of phosphate ore tailings, 93-98 mass% sulfuric acid, and acidic wastewater of phosphoric acid factory is 100:50-90:100; controlling the stirring speed at 200-500 revolutions/minute; and reacting for 10-30 minutes at 30-60 degrees C;    (B) baking the obtained product at 80-100 degrees C until water content is 1-3%; piling up at room temperature; and crushing into powder composite fertilizer filler; and    (C) mixing the powder product with urea, ammonium phosphate, potassium sulfate, or potassium chloride; sending to a pan granulator; adding with water; pelletizing; drying; sieving to obtain 1-4 mm particles of greater than or equal to 90%, where granulation rate is greater than or equal to 70% and particle strength is greater than or equal to 25 N; and obtaining granulated composite fertilizer.