The invention provides a robot smart arm joint driver and a decelerating device thereof, consisting of a flange, a motor fixed frame, an end cover, a deep ditch ball bearing, a permanent magnetic DC moment motor, a hexagon thin nut, a spring washer, a sleeve 1, a bearing retaining cover, a second sleeve, a harmonic reducer, a through shaft, a rolling needle bearing, a thrust rolling needle bearing and a connecting flange. The permanent magnetic DC moment motor is fixed on the motor fixed frame; the rotor of the motor is connected with the connecting flange which is connected with the harmonic generator of the harmonic reducer; the steel wheel of the harmonic reducer is fixed on the motor fixed frame by a screw; the flexible wheel of the harmonic reducer is connected with the through shaft and the flange; the rolling needle bearing is arranged between the rotor of the motor and the harmonic generator and the through shaft; the deep ditch ball bearing is arranged between the flange and the motor fixed frame. The joint driver and the decelerating device have the advantages of compact structure, smart motion, simple control, good application effect and great implementation value.