The darkening of baby teeth may be a manifestation of tooth decay or exogenous staining of teeth. The appearance expression of tooth decay is tooth darkening, and the common clinical one is bottle caries, when drinking milk, the bottle is attached to the lip surface of maxillary teeth, and the milk or sugar in the bottle stays on the surface of teeth, bacteria use these substances as substrate to ferment and produce acid, resulting in tooth decay. Bacteria infect the hard tissue of the tooth with a black appearance. It is necessary to apply fluoride or tooth filling treatment as soon as possible. In addition to the cause of caries, the black coloring of baby teeth may also be an exogenous coloring of teeth. Babies like to eat colored food, such as chocolate or Oreo cookies, which will cause pigmentation on the tooth surface, and the teeth will be in a dark colored food for a long time, causing a kind of tooth coloring, which can be removed by brushing. If a bacterial infection occurs and the teeth become demineralized, food debris or pigment enters the tooth tissue, the appearance of the teeth is difficult to remove by brushing and a dental filling is required. The darkening of baby teeth may also be caused by necrosis of the nerve. The tooth loses the nutritional function of the pulp tissue and will appear black and dark, which is a manifestation of a dead pulp tooth and requires root canal treatment of the tooth. Therefore, the darkening of baby teeth may be the result of tooth staining or tooth decay, which requires early medical attention.