The invention relates to a method for preventing the cast tooth surface of a large cast tooth casting generating casting defects. The invention comprises mold fabrication, modeling, mould closing, casting, casting mold cooling, casting cleaning and heat treatment. The casting technique parting surface of the casting mold is arranged at the upper plane of the cast tooth, the cast tooth of the casting is positioned at a lower half casting box, a ring-shaped slag collecting groove connected with all cast teeth is arranged above the cast tooth addendum circle of a sand mould parting surface, and multi riser runners are arranged at the upper part of the slag collecting groove. By adopting the method of the invention, the cast tooth surface of the large casting such as a disk wheel can be prevented from generating a slag blowhole, the material compactness of the casting can be improved, and precision of the tooth profile of the cast tooth can be improved. The method of the invention has simple modeling operation, reduces workload of casting cleaning and polishing and processing, improves production efficiency and reduces production cost.