The invention belongs to the field of coatings, and relates to a method for increasing the thickness of an outer enamel layer of ceramsite by using sludge slurry. The ceramsite is prepared by the following steps: mechanically grinding three materials including sludge, fly ash and coal gangue for 1-2 hours respectively, and sieving the three materials through a 100-mesh sieve; uniformly mixing thethree materials in parts by weight for 20-30 minutes, and drying the mixture in a blast drying box at the temperature of 90-105 DEG C until the mass is constant; carrying out pressure molding by usinga tablet press under the condition that the molding pressure is higher than 3 MPa to obtain a raw material column type compound green body; mixing the sieved sludge material with water in a certain proportion to prepare sludge slurry; placing the green body in the sludge slurry for a short time to wet the green body, and uniformly coating the green body with the sludge slurry for sintering; sintering the green body at an air flow rate of 50-100 ml/min at the temperature of 1000 DEG C-1250 DEG C, and cooling the sintered product to the room temperature after sintering is finished to obtain themolded ceramsite. The thickness of the enamel layer on the outer layer of the ceramsite product can be effectively increased by coating the ceramsite with the sludge, and the enamel layer on the outer layer of the sintered ceramsite product is smooth and flat.