The appropriate amount of fluoride can prevent caries, and the use of fluoride includes both local and systemic application. Systemic application: fluoride water source, home drinking water fluoridation, oral fluoride (fluoride tablets, fluoride containing solution, tea drinking, etc.). Topical application: fluoride solution application, fluoride gel, fluoride foam, fluoride toothpaste, fluoride dental floss, etc. Topical application of fluoride is an effective caries prevention method, the mechanism of which is that fluoride can enhance the acid resistance and remineralization ability of enamel surface. Due to the convenience and safety of using fluoride foam, it has been widely used in children in kindergartens and schools, and in addition, it has been gradually and widely used in adults as well as people with high risk of caries rate, such as fixed orthodontic patients. However, excessive fluorine intake can cause acute or chronic fluorosis. Therefore, topical fluoride should be applied regularly by a professional dentist and should not be used too heavily or too often. Enamel demineralization and even dental caries caused by fixed braces is also a noteworthy problem and necessary measures should be taken for prevention. One of the important preventive measures is to apply low concentration of fluoride foam coating to treat the tooth surface during the orthodontic treatment, which can effectively prevent and reduce the occurrence of enamel demineralization.