The invention discloses a traction-type robot speed reducer. The traction-type robot speed reducer comprises a shell body and a planet speed reduction mechanism arranged in the shell body, wherein theplanet speed reduction mechanism comprises an outer ring and a planet carrier arranged in the outer ring; a sun rolling wheel located at the center of the planet carrier and planet rolling wheels arranged at the two sides of the planet carrier are arranged on the planet carrier; the outer surfaces of the planet rolling wheels make contact with the inner surface of the outer ring and the outer surface of the sun rolling wheel separately; the planet rolling wheels and the extension line of the contact line of the outer ring and the sun rolling wheel are intersected at one point separately, andthe point is located on the axis of the sun rolling wheel; and an end-surface cam loading mechanism used for being matched with the end surface of the outer ring is arranged in the shell body. The traction-type robot speed reducer is capable of realizing zero-spin transmission, ultra-high in accuracy, and capable of carrying out automatic adjustment for pre-tensioning loading according to a load,so that the life of the traction speed reducer is prolonged, the accuracy requirement of vibration and assembly is lowered, and stable running is realized.