Secondary caries is most likely to cause secondary caries in children’s dentistry, and the reasons for its development are as follows: 1. It is related to the cavity preparation without exhausting the decayed tissue, which causes the caries to continue to develop after filling. At the same time, it is related to whether the cavity wall has base enamel or not when the operator operates to prepare the cavity, especially in the place that bears the biting force, the teeth are easily broken when the force is applied, leaving a gap at the edge of the cavity, which is conducive to the deposition of plaque; the edge of the cavity is in the retention area or at the deep cavity, and it is related to the micro-leakage between the interface of the filling material and the cavity wall at the same time. At the same time, when the filling material is hardened, the volume contraction of itself is smaller than the thermal expansion coefficient of the hard tissue of the tooth, and the microleakage between the cavity wall and the filling material can be caused by corrosion, insufficient filling pressure and dissolution of the cushion adhesive at the edge of the cavity, etc.; 2, secondary caries is also related to the anatomical form, tissue structure, mineralization degree and the environment of the milk tooth.