An electric grinding gun has a body, a gear reduction device, a holding cap, and an actuating device. The body has a casing, a rechargeable battery, a protective circuit board, a switch, a trigger, a motor, a cooling fin and a mounting barrel. The gear reduction device is rotatably mounted in the mounting barrel and has a seat, two planet gears and a gear ring. The seat is rotatably mounted in the mounting barrel and has a shaft hole, two gear recesses and a connecting axle. The holding cap is securely mounted around the mounting barrel to hold the gear reduction device in the mounting barrel and has an inner thread and a through hole. The actuating device is connected detachably to the seat of the gear reduction device and has a panel and an adhering layer.