The invention discloses a method and device for preventing casting of a discharging port of an extruding machine. The device comprises a jet nozzle, wherein the jet nozzle has an extruding cavity, anda valve element base, a movable valve element and a Belleville spring are arranged in the extruding cavity. The Belleville spring is arranged at the bottom of the extruding cavity and is provided with the movable valve element, the bottom surface of the movable valve element is pre-compressed and arranged at the upper end of the Belleville spring, and the top face of the movable valve element isprovided with a shar end. The middle position of the top face of the valve element base is provided with a round hole, the round hole is communicated with the extruding cavity, the bottom surface of the valve element base is compressed on the sharp end of the movable valve element. The inner walls of the two sides of the extruding cavity are provided with inner threads, the sides, close to the inner threads, of the valve element base are provided with outer threads, and the valve element base is in screwed connection with the extruding cavity. The device is simple in structure, is smaller in volume compared with a needle-type valve having the same functions and is smaller in production difficulty, the needed heating power is lower when the device is used, meanwhile the when the casting andwiredrawing phenomena of the jet nozzle when extruding machine completes extruding work are effectively avoided, the quality of a product is ensured, and the raw material cost is also saved.