This is a placebo controlled, randomized, double blinded study including Phase 1 and Phase 2. Phase I study is a safety assessment and Phase 2 study is incline to assess effectiveness of MSCs. Potential subjects must be screened and consented before enrolled. The primary objective of this study is to determine the effects of early intravenous infusion of allogeneic human umbilical cord mesenchymal stem cells (HucMSCs or MSCs used in the following section) for patients with acute ischemic stroke. Eligible patients will receive a single dose of MSCs or placebo within 24 hours after stroke. Patients will be followed for 2 years post infusion for safety and efficacy (change in neurological symptoms and quality of life). Assessments will occur during transplantation and at 3,7, 14 days and1,3, 6, 12, 18 and 24 months after infusions of stem cells.