The invention relates to the technical field of speed reducers and discloses a robot speed reducer with an excellent heat dissipation effect. The robot speed reducer which comprises a bottom plate, wherein a housing is fixedly mounted at the top of the bottom plate; a cover plate is fixedly mounted at the top of the housing; and a support block is fixedly mounted in the housing and at the top of the bottom plate. According to the robot speed reducer, through cooperative application of a ventilation opening, a grating plate, a vent pipe, an electric motor, a rotary shaft, a fan blade, a heat dissipation block, a heat dissipation plate, a cooling box, a condensation box, an evaporation pipe, a condensation plate, a flow guiding plate, a cooling liquid return pipe, a heat dissipation hole, adustproof net, an air passing hole and a heat dissipation groove, heat in the housing can be rapidly transmitted to the external part during the operating process of the robot speed reducer, so that the internal temperature of the housing is prevented from being excessively high to avoid influence to normal operation of the robot speed reducer. The robot speed reducer has excellent heat dissipation performance, and is longer in service life and better in using effect.