The invention relates to the field of mechanical transmission, in particular to a harmonic gear reducer which comprises a wave generator, a flexible gear, a steel gear, a fixing mechanism, an annular air bag, a block mechanism and a shaft, the outer wall of the wave generator is connected with the flexible gear in a clamped mode, an outer gear of the flexible gear is meshed with an inner gear of the steel gear, and the steel gear is fixedly connected with the fixing mechanism. A block mechanism is arranged in the fixing mechanism, an annular groove is formed in the outer wall of the block mechanism and matched with an annular air bag, a threaded hole is formed in the middle of the block mechanism, the block mechanism is in threaded connection with a shaft, the shaft is fixedly connected with a wave generator, and the wall thickness of the position a in the annular air bag is larger than that of the position b in the annular air bag. The thickness of the wall at the position a is larger, the abrasion resistance of the annular air bag can be improved, the service life of the annular air bag can be prolonged, the air holes are formed in the position b of the annular air bag, the hole diameters of the air holes are gradually reduced from inside to outside, the air flow speed can be increased in the air spraying process, and therefore heat loss is accelerated, and the dust removal effect is achieved.