BACKGROUND: Methylmalonic acidaemia (MMA) is a genetic metabolic disorder caused by congenital defects that may result in multisystem damage. However, MMA complicated with acute myeloid leukemia (AML) is very rare. CASE PRESENTATION: Here, we report a case of MMA with AML in a boy aged 7 years and 6 months. The boy was screened for MMA after birth and received long-term treatment with dietary control, vitamin B12, and L-carnitine. He was readmitted at the age of 7 years and 6 months with systemic bleeding spots and was diagnosed with AML by bone marrow cytology. When the diagnosis was clear, the patient received chemotherapy in addition to maintenance treatment for MMA. His blood routine, liver and kidney function, blood biochemistry, blood glucose, blood lipids, lactic acid and blood ammonia were monitored continuously. Bone marrow cytology one month after starting chemotherapy showed complete remission. Sorafenib was also added during chemotherapy and was discontinued 1 year after completion of chemotherapy. At the end of chemotherapy, he chose venetoclax for consolidation, and to date, the patient's condition is stable, with sustained CR and no relapse. CONCLUSION: MMA combined with AML is rare. The prevention of infection, comprehensive assessment of nutritional status, continuous monitoring of related indicators and overall situation should be prioritized and comprehensive and individualized treatment approaches are paramount.