Inclusion Criteria: * Six months of age and younger on day of vector infusion with Type 1 SAM as defined by the following features: * Diagnosis of SMA based on gene mutation analysis with bi-allelic SMN1 mutations (deletion or point mutations) and 2 copies of SMN2; * Onset of disease before 6 months of age * The patient's legal guardian(s) must be able to understand the purpose and risks of the study and voluntarily provide signed and dated informed consent prior to any study-related procedures being performed. Exclusion Criteria: * Patient who has participated in a previous gene therapy research trials; * Patient who has received Nusinersen and Risdiplam treatment; * Patient who has AAV9 neutralizing antibody titer ≥1:200; * Patient who requires non-invasive ventilatory support averaging≥16 hours/day; * Patient with a point mutation in SMN2 (c.859G\>C); * Patient who requires non-invasive ventilatory support averaging≥16 hours/day at screening; * Patient who use invasive ventilatory support or pulse oximetry \< 95% saturation while awake and calm at screening; * Patient who is positive for human immunodeficiency virus (HIV) antibody, hepatitis B surface antigen, hepatitis C antibody, or treponema pallidum antibody; * Abnormal laboratory values considered clinically significant, including gamma-glutamyl transferase(GGT), Aspartate aminotransferase (AST), alanine aminotransferase (ALT), bilirubin \> 3x upper limit of normal (ULN), Hemoglobin (Hgb)\< 110 or \>150 g/L, platelet \<183x10\^9/L or 614x10\^9/L; * Class IV patient based on Modified Ross Heart Failure Classification for Children; * Patient with a history of glucocorticoid allergy; * Contraindication that would interfere with the lumbar puncture procedures; * Presence of an untreated active infection requiring systemic antiviral therapy at any time during the screening period; * Vaccination less than 2 weeks before infusion of vector; * Patient who has any concurrent clinically significant major disease or any other condition that, in the opinion of the Investigator, makes the subject unsuitable for participation in the study. Note: Other protocol defined inclusion/exclusion criteria may apply.