The invention relates to the technical field of machining, and particularly discloses a device for machining the conical surface of a circular mold discharging hole. The device comprises a pedestal, amachining platform arranged on the pedestal, a driving mechanism used for driving the machining platform to move longitudinally on the pedestal, a drill bit arranged on a supporting rod, a driving mechanism of the drill bit and an electrical PLC used for controlling all the driving mechanisms. A first servo motor and rotating shafts driven by the first servo motor to rotate are arranged on the machining platform. The rotating shafts are distributed in the longitudinal direction of the machining platform. The machining platform is further provided with a driven shaft parallel to the rotating shafts. By means of the device for machining the conical surface of the circular mold discharging hole and the method thereof, the drill bit and the circular mold discharging hole can be automaticallycentered quickly, and therefore rapid chamfering can be achieved, the control automation degree and precision are high, and automation and integration can be realized conveniently; and the chamferingspeed of a circular mold is high, the precision is high, the degree of automation is high, chamfering strength is greatly reduced, chamfering efficiency is improved and machining benefits are increased.