The reasons for long black spots on teeth are pigmentation, caries damage, and heavy fluorosis, as follows:1, the restoration is not polished: when applying composite resin and other repairs to the labial defects of the teeth, if the surface is not polished, pigments from food and drinks will be deposited on the surface of the filling body, producing black plaques.2, caries damage: when suffering from enamel dysplasia or mineralization incompleteness, the surface of the teeth will form a substantial defect, which often Heavy dental fluorosis: when suffering from heavy dental fluorosis, it will lead to yellowish-brown and blackish-brown plaques on the lip surface of the teeth, which affects the aesthetics.4. Tetracycline teeth: when tetracycline teeth occur, black plaques will appear on the surface of the teeth, and at the same time, the combination of enamel and dentin is more loose, and a large enamel peeling can occur.5. Exogenous pigmentation: when there are habits such as smoking, tea and coffee, the exogenous pigments will deposit on the surface of the filling body to produce black plaques. Exogenous pigmentation can be deposited on the surface of the teeth when you smoke, drink tea or coffee, etc., forming large black spots.