BACKGROUND Osteopetrosis is a family of extremely rare diseases caused by failure of osteoclasts and impaired bone resorption. Among them, autosomal dominant osteopetrosis type II (ADO II), related to the chloride channel 7 (CLCN7) gene, is the most frequent form of osteopetrosis. In this study, we report a de novo mutation of CLCN7 in a patient without the family history of ADO II. CASE SUMMARY A 5-year-old Chinese boy with ADO II was found to have a de novo mutation in the CLCN7 gene [c.746C > T (p.P249L)]. Typical clinical manifestations, including thickening of the cortex of spinal bones and long bones, non-traumatic fracture of the femoral neck, and femoral head necrosis, were found in this patient. The patient is the first reported case of ADO II with the missense mutation c.746C > T (p.P249L) of the CLCN7 gene reported in China. We also review the available literature on ADO II-related CLCN7 mutations, including baseline patient clinical features, special clinical significance, and common mutations. CONCLUSION Our report will enrich the understanding of mutations in ADO II patients. The possibility of a de novo mutation should be considered in individuals who have no family history of osteopetrosis.