The invention belongs to the technical field of a reducing device used for an industrial locomotive, in particular to a reducing device used by a narrow-track industrial locomotive of 20 tons, which solves the problem of incapable of meeting the traction with small track distance and large torque of the existing reducing box. The reducing device used by the narrow-track industrial locomotive of 20 tons comprises a box body which consists of an upper box body and a lower box body; a transmission part is arranged in the box body; the transmission part consists of two grade transmission of a full-closed cylindrical helical gear and a spiral conical gear. A small helical gear shaft, a small spiral conical gear shaft and a vehicle shaft are arranged on the box body by a supporting bearing; a bearing cover is arranged outside the supporting bearing; an adjusting spacing ring is arranged between the bearing and the bearing cover; the surrounding of the adjusting spacing ring is provided withoil leakage holes; the bearing cover arranged at the vehicle shaft is an embedded structure. The reducing device used by a narrow-track industrial locomotive of 20 tons has the beneficial effects of large transfer torque, small volume, stable transmission, being quiet, low fault rate, large intensity and rigidness of the box body, fully meeting the large power transmission under a small track distance and being capable of being widely applied to the transmission reducing devices of the locomotives used for mine and tunnel constructions.