The invention relates to a mortar spraying device, which comprises a rack, a feeding hopper, a conveying auger, a stirring bin, a stirring shaft, a screw pump, a spraying hose and a spraying head, wherein the conveying auger is arranged on the rack; an outlet of the feeding hopper is connected with a charging hole of the conveying auger; a discharging hole of the conveying auger corresponds to the charging hole of the stirring bin; the stirring bin is formed by connecting two cylinders which are coaxial and have different inner diameters and arranged on the rack; a water inlet hole is formed in the upper part of the stirring bin; the stirring shaft is arranged in the stirring bin by using a pair of bearings; a plurality of stirring rods are arranged on the stirring shaft; stirring blades are arranged at one end of the stirring rods; the charging hole of the stirring bin is formed in the side wall of one end, of which the inner diameter is larger, of the stirring bin; the discharging hole of the stirring bin is formed in the side wall of one end, of which the inner diameter is smaller, of the stirring bin; the discharging hole of the stirring bin is connected with the charging holeof the screw pump through a pipeline; and the discharging hole of the screw pump is connected with the spraying head through the spraying hose. By adoption of the mortar spraying device, the quality and efficiency of the wall surface spraying operation of a fireproof and waterproof material can be greatly improved.