Objective To investigate the therapeutic effects of transplanting allogeneic marrow mesenchymal stem cells (MSCs) via subarachnoid space on spinal cord injury(SCI) and the T cell subpopulation. Methods Density gradient centrifugation was used to isolate and expand MSCs from bone marrow of 10 six-week-old SD rats. The SCI model was produced by weightbeating from 60 eight-week-old female SD rats. Forty survival SCI rats,which BBB scores were zero, were divided randomly into 2 groups:experimental group(group A) and control group(group B). In addition, 20 normal eightweekold SD ratswere used as blank group (group C). In group A, 1 ml cells suspention containing MSCs(the 6th generation, 2×106/ml) was injected via subarachnoid space. Ingroup B, equal volume of L-DMEM was injected in the same way. The BBB score was obtained after 1st,2nd and 3rd weeks of injection. At the same time,T cell subpopulation was detected by flow cytometry. Results The BBB score in group A was better than that in group B, but fewer than that in group C in the 3rd week. CD4+T cells in group A were less than those in groups B and C in the 1st, 2nd, and 3rd weeks. CD8+T cells in group A were less than those in groups B and C in the 2nd and 3rd weeks. The ratio of CD4+/CD8+T cells in group A was less than those in groups B and C in the 1st week. Above differences showed statistically significant difference(P＜0.05). However, there were no statistically significant differences in the ratio of CD4+/CD8+T cells between group A and groups B, C in the 2nd and 3rd weeks (P＞0.05). Conclusion The above results suggest that allogeneic MSCs transplantation via subarachnoid space is beneficial to SCI to some extend, do not result in rejection in vivo. Furthermore, it can lead to immunosuppression in short time. So, it provides clues to apply MSCs to treat SCI and other diseases.