Clinically, many parents are not clear about when to do the fossa closure for their children, sometimes they come to the clinic and find that their children’s teeth have caries, now we make a brief introduction to the timing of fossa closure: Fossa closure is to use special fluorine-containing resin-like material fossa sealant to fill the deeper fossa and furrow on the surface of the molar, and after light curing, the fossa point gap on the tooth surface is closed to form a layer of protective barrier. The first time is at the age of three or four when the milk molar erupts; the second time is at the age of six when the first permanent molar erupts; and the third time is at the age of twelve when the second permanent molar erupts. Many parents think that the milk molar will be replaced by the permanent molar sooner or later, and it doesn’t matter if it grows caries, which is a wrong concept, because the development of milk molar is directly related to the arrangement of permanent teeth, and even affects the child’s facial development. The chance of children suffering from dental caries is greatly reduced.