Abstract: By using the tail suspension rat model, the effects of the simulated microgravity on T cell development in thymus and the Neuroimmunologic mechanism were explored. The changes of different subtype T lymphocytes in thymus of the tail suspension rat were tested by flow cytometry detection. The content of norepinephrine (NE) and endogenous glucocorticoid (GC) in rat thymus tissue were detected by ELISA tests. Obvious atrophy of thymus organ could be found in tail suspension rats (p<0.05). The data of flow cytometry analysis show that the CD3⁺CD4⁺CD8^- and CD3⁺CD4^-CD8⁺ T cell count decreased significantly in tail suspension rats thymus compared with control group (p<0.05). The ELISA assay showes that the content of NE and GC under simulated microgravity has a tendency to rise. The blocking of CD3⁺CD4⁺CD8⁺ T cells develops to CD3⁺CD4⁺CD8^- and CD3⁺CD4^-CD8⁺ single positive T cells in tail suspension rat thymus, which might associate with neuroendocrine regulation.