Objective　To explore the clinical features of Megalencephaly-capillary malformation-polymicrogyria syndrome (MCAP) caused by PIK3CA gene mutation. Method　The clinical phenotype, imaging findings and follow-up of MCAP caused by missense mutation of PIK3CA gene in a child were retrospectively analyzed, and the related literature was reviewed. Results　A girl aged 4 years and 2 months can only say ma, pa and understand simple instructions. She had unsteady walking gait and was easy to fall. The child had a head circumference of 56.9 cm, a high palatal arch, and a sinus tract in the left auricle. The left limb was more hypertrophic than the right side and the lower limb joints were over-extended. She had low muscle tension and the muscle strength was grade IV. Gessell development scales showed the development quotient was 32. Cranial magnetic resonance imaging showed type I chiari malformation with obstructive hydrocephalus. High-throughput exon sequencing results showed that PIK3CA gene had a "missense mutation of c.1133G>A, p.Cys378Tyr (heterozygous)", which was a new mutation. The parents of the children had normal genotypes. The child was followed up to the age of 5 years and 1 month. She could speak short sentences of 3-4 words and communicate in simple language. She was still unsteady in walking and easy to fall, with a head circumference of 57.7 cm. Conclusion　A case of MCAP caused by a new mutation of PIK3CA gene was found, which enriched the mutation spectrum of PIK3CA gene.