Abstract: Multiple acyl-CoA dehydrogenase deficiency (MADD) is a rare inherited metabolic disorder of fatty acid oxidation, with highly heterogeneous clinical manifestations. Late-onset MADD predominantly presenting with myopathic features is easily confused with inflammatory myopathy; early identification and administration of high-dose vitamin B₂ can achieve favorable therapeutic effects. This article reports a MADD patient with main manifestations of exercise intolerance and progressive post-exertion muscle weakness. The case is analyzed by integrating blood and urine metabolic screening results and genetic testing findings, so as to provide a reference for the early diagnosis of this disease.