Accommodative insufficiency (AI) is common in children, however, has not been described in Prader-Willi syndrome (PWS). This case report presents severe AI in a child with PWS and a rare mutation on chromosome 15 (methylation at locus SNRPN). A 15-year-old boy with PWS presented with the complaint about needing to remove distance glasses while reading. The visual acuity in his right eye was 20/20 with -2.0 D, and in his left eye 20/20 with -2.75/-0.25/173°. The defocus curve manifested with severe AI, and no other abnormal ocular findings were noted. Progressive glasses were recommended. Molecular genetic analysis at the age of two years revealed altered methylation at locus SNRPN on chromosome 15. As muscular hypotonia is common in PWS, the function of smooth muscles, including the ciliary muscle might be altered, as demonstrated in this case report.