Objective　To explore the clinical and genetic characteristics of congenital melanocytic naevi (CMN). Method　The clinical data of CMN in a child was retrospectively analyzed. Results　HE staining of damaged skin tissue in a 5-year-old boy showed a large number of melanocytes were widely distributed in the dermis and he was diagnosed of CMN. The chromosome karyotype analysis of the damaged skin tissue showed no significant abnormalities. The results of gene detection showed a heterozygous mutation of NRAS gene, C. 182A > G (Q61R), in the damaged skin tissues. Pedigree analysis showed that the correspondent loci in the peripheral blood of the child and his parents were wild-type. Conclusions　CMN was caused by somatic mutation of NRAS gene in this case, and genetic test of damaged skin tissue was helpful for diagnosis and prognosis of CMN