Objectives: To investigate the clinical and genetic characteristics of children with glycogen storage disease type IIIa (GSD IIIa) and to explore the muscle involvement and manifestations of GSD IIIa patients. Methods: The clinical data of 11 patients with GSD IIIa diagnosed by genetic testing from 2003 to 2019 were retrospectively analyzed. Results: Twenty variants of AGL gene were detected in 11 patients, eight of which were novel variants. Before treatment, the height was significantly backward. All patients had hepatomegaly. Abnormal biochemical indicators were mainly manifested as significantly increased serum liver and muscle enzymes, accompanied by hyper-triglyceridemia, hypoglycemia, hyperlactacidemia, slightly elevated pyruvic acid, and metabolic acidosis. After treatment, the height and liver size of the patients were significantly improved. At the same time, alanine amino-transferase (ALT), aspartate aminotransferase (AST), triglyceride (TG), lactic acid and pyruvic acid in children were significantly decreased, while creatine kinase (CK) was significantly increased. During follow-up monitoring, six patients developed ventricular hypertrophy. Lactate dehydrogenase (LDH) (691.67 +/- 545.27 vs. 362.20 +/- 98.66), lactic acid (3.18 +/- 3.05 vs. 1.10 +/- 0.40), and pyruvic acid (64.30 +/- 39.69 vs. 32.06 +/- 4.61) were significantly increased in patients with ventricular hypertrophy compared with those without ventricular hypertrophy. Conclusions: In clinical cases of upper respiratory tract infection or gastrointestinal symptoms accompanied by hypoglycemia, dyslipidemia, metabolites disorders, elevated serum liver, and muscle enzymes, the possibility of GSD ilia should be vigilant. During treatment monitoring, if lactic acid, pyruvic acid, LDH, and CK rise, it indicates that the disease is not well controlled and there is the possibility of cardiac hypertrophy.