Hereditary spinocerebellar ataxia (SCA) is a devastating, incurable disease. Stem-cell-based therapies represent new promise for clinical research in neurology. The objectives of this study were to assess the feasibility, efficacy, and potential toxicity of human umbilical cord mesenchymal stem cells (UCMSCs) therapy in patients with SCA. Sixteen genomically diagnosed SCA patients were enrolled and received intravenous and intrathecal infusion of UCMSCs. Clinical, laboratory, and radiographic evaluations were conducted to assess the safety of UCMSC therapy. Efficacy was evaluated by the Berg Balance Scale (BBS) and International Cooperative Ataxia Rating Scale (ICARS) scores. Among the 16 cases, there were no serious transplant-related adverse events happened in 12 months follow-up. The majority of patients showed improved BBS and ICARS scores continuing for at least 6 months which indicated UCMSC therapy could alleviate SCA symptoms. This study suggested that UCMSC transplantation was safe and might delay the progression of SCA. This may represent a new therapeutic strategy for SCA and other genetic neurological diseases.