Objectives: Sanfilippo syndrome (Mucopolysaccharidosis III, MPS III) is a rare autosomal recessive hereditary disease, which is caused by lysosomal enzyme deficiency. This study was operated to investigate clinical and molecular characteristics of patients with MPS III, which will improve the diagnosis and treatment of MPS III. Method: Thirty four patients with MPS III were assessed using clinical evaluation, questionnaire, and scoring system. Results: Among the 34 patients, 14 had MPS IIIA, 19 had MPS III B, and one had MPS III C. Speech delay (100%) and intellectual disability (100%) were the most prevalent clinical manifestations in this cohort, followed by hyperactivity (94.12%), hirsutism (91.18%), enlarged head circumference (73.52%), repeated diarrhea (67.64%), sparse teeth (67.64%), and Mongolian spots (64.71%). There were two clinical manifestations that were significantly different between IIIA and IIIB: Hepatosplenomegaly and serrated teeth. The most common initial symptoms at diagnosis were speech delay (52.94%), hyperactivity (35.29%), and mental retardation (29.41%). Genetic analysis of 25 patients was conducted, which identified 12 novel mutations. Conclusion: When language retardation, mental retardation, and rough facial features occurred, MPS III should be considered. At same time, more examination should be operated, such as examination of changes in cranial magnetic resonance imaging of cerebral cortex atrophy. Hepatosplenomegaly and serrated teeth could be used clinically to preliminarily distinguish IIIA from IIIB.