Down syndrome is the most common autosomal disorder, also known as trisomy 21, congenital stupidity. It has a prevalence of 1 in 600-800 and a child with Down syndrome can be delivered to a pregnant woman of any age, but generally increases with the age of the mother. A child with Down syndrome has trisomy 21, which means that he has an extra chromosome 21. It has three karyotypes, the majority of which are standard, about 95%, it is 47, XY (or XX), +21, translocation about 2.5~%-5%, and a chimeric type, which is about 2~%-4%. Once a child is suspected of having trisomy 21, a karyotype analysis should be done promptly, especially for newborns. Children with Down syndrome often have typical facial features: small eye fissures, wide eye spacing, upward sloping external canthus of both eyes, internal capital redundancy, low and flat nasal bridge, small external ears, a narrow hard palate, so the child often opens the mouth and stretches out the tongue, and has more halitosis, a small and round head, a large fontanelle with delayed closure, a relatively short and wide neck, and a dull expression. The child may have typical through hands and straw shoe feet. There are varying degrees of mental retardation. About 50% of the children have congenital heart disease and gastrointestinal malformations. The incidence of congenital hypothyroidism and acute lymphoblastic leukemia is also significantly higher than normal. When a child has the typical facial features described above, a karyotype analysis of the chromosomes should always be done. To avoid the birth of a child with Down’s syndrome, screening for Down’s syndrome should be performed during pregnancy so that the birth of a child with Down’s syndrome can be avoided to the maximum extent possible.