A machining device for duplex gear of high-precision reducer for robot includes a laser welding device, a laser rotary support, a rotary mechanism and a rotary platform, the rotary mechanism being installed with a reducer body, wherein a center of a central gear is fixedly connected with a rotating shaft; sets of duplex gears are evenly disposed around the central gear; the gear of each set of duplex gear which is connected with the central gear are connected to an output gear ring from outside; a fixed gear ring is disposed above the output gear ring; a rotating frame is disposed at ends, close to the top of the reducer, of rotating shafts of the sets of duplex gears and the rotating shaft of the central gear.