The invention discloses a rotary speed reduction device for an automobile crane, which mainly comprises a motor flange, a brake, a planetary gear reducer, an output gear, a bearing and a bearing pedestal, wherein the brake comprises a shell, a brake piston, a brake spring, kinematic/static friction plates, a combined seal, a gasket and a retainer ring; the planetary gear reducer is transmitted by a three-stage planetary gear train composed of a spline shaft sleeve, big, medium and small sun gears, big, medium and small planetary carriers, big, medium and small planetary gears, big and small internal gears and the like; and the output gear is supported in the bearing pedestal through the bearing, the upper part of the output gear is provided with a spline shaft and is connected with an internal splined hole of the big planetary carrier, and the lower part of the output gear is provided with an external gear. The rotary speed reduction device adopts three-stage planetary gear speed reduction to effectively improve a reduction ratio and the speed reduction and torque increase capability, improve the brake efficiency and the brake reliability, fulfill the aim of high efficiency and low-noise transmission, and effectively solve the problems of the conventional rotary speed reduction device used in the automobile crane. The rotary speed reduction device has the advantages of simple structure, large reduction ratio, high speed reduction and torque increase capability, and high sealing property.