The number of children in western countries diagnosed with Down syndrome (DS), a disease caused by chromosomal abnormalities, is still increasing. The resulting delayed cognitive development also leads to deficits in social functioning in children under 12. Active music therapy (MT), as a natural intervention to achieve therapeutic functions through improvisation, performance, and singing, has been proven to promote the socialization development of children with DS. To get a better idea of the extent of the promotion and exactly what aspects it has improved, I conducted literature reviews and interviews with a music therapist. The results suggest that active MT can stimulate the socialization development of children under 12 with DS in three aspects: language skills, social-emotional development, and prosocial behavior. Although the improvement effect of active MT is better than that of passive MT, it requires the client to have a basis of music theory, that is, it requires higher requirements on the client, so the treatment method should be selected according to the severity of the client's DS or a combination of the two. Hence, future research could be aimed at finding the simplest interventions with sufficient client engagement and verifying the continuation of the effect of active MT.