Tooth decay belongs to the chronic hard tissue infection of teeth, and it takes 0.5-1 years from the first black spots on the tooth surface to the formation of the final cavity. Due to the lack of good oral hygiene, a lot of food residues accumulate on the tooth surface, the bacteria on the tooth surface will decompose the food residues to produce acid, and the acid will cause the demineralization and softening of the tooth enamel after staying on the tooth surface for a long time, which will lead to the defect of the tooth and finally form the cavity. Once the defect of the cavity breaks through the enamel and reaches the dentin, because the hardness of the dentin is not as hard as the enamel, so the caries will progress very fast in the dentin, and once the cavity appears, the cavity itself is the place where the dirt is hidden and hard to clean, so it will further accelerate the expansion of the cavity, thus the cavity will easily cause the pulp infection and cause the severe pain caused by pulpitis. Therefore, we must brush and rinse our teeth carefully everyday, and if black spots or superficial cavities appear in the mouth, we must seek medical treatment as soon as possible to prevent further expansion of cavities to cause more serious tissue damage.