Mucopolysaccharidosis type I (MPS I) is a rare autosomal recessive inherited lysosomal storage disorder, caused by a deficiency or complete absence of enzyme alpha-L-iduronidase（IDUA），leading to accumulation of the glycosaminoglycans (GAGs) dermatan and heparan sulfate in organs and tissues, which results in series of clinical manifestations, including arthropathy commonly. If untreated in time, patients may suffer from irreversible cognitive impairment, even die within the ?rst decade of life with the severe phenotype. Therefore, early diagnosis and treatment are crucial to improve the prognosis. In this study we described a MPS I boy caused by IDUA homozygous mutation misdiagnosed as juvenile idiopathic arthritis (JIA) , helping rheumatologists to improve the knowledge of this disease and extend the thought of differential diagnosis of arthritis.