The time of formation of secondary caries is related to the cause of its development. Due to the medical factor, i.e. the previous doctor did not remove the decayed tissue, and the secondary caries actually existed at the moment when the material was filled up. Except for the medical factor, if the original filling material falls off, it takes 2-3 years to develop the caries again; if it is caused by the material aging, it is 5-10 years to cause the secondary caries in general. Clinically, secondary caries is a rare phenomenon, and most of the patients will not have secondary caries after filling the affected teeth. If the patient goes to a regular unit for treatment, the doctor will carry out the standardized operation and there is no need to worry about the secondary caries. If the secondary caries cannot be avoided, the main thing is to keep the oral hygiene and reduce the bacterial accumulation, which can also prevent or stop the further development of secondary caries.