Objective：To observe the differentiation of bone marrow mesenchymal stem cells (BMSCs) induced by astragaloside A and expression of Cyclin D1. Methods: BMSCs were induced by astragaloside A with 100 mg·L-1. At l, 12, 24 h after induetion, the morphological changes of BMSCs were observed with optical microscope and the expressions of Nestin, NSE, Cyclin D1 were identified by immunocytochemical techniques. Cyclin D1 mRNA was detected by RT-PCR．Results： Bone marrow cells were primarily cultured and subcultured. The third passage of BMSCs was purified. After treatment with 100 mg·L-1 astragaloside A, some cells presented processes, were similar to neuron-like cells in morphology, and expressed Nestin and neuron-specific enolase. Nestin expression occurred earlier (P<0.05) and at a higher level (P<0.05) than expression of neuron-specific enolase. At 1 hour after treatment with astragaloside A, cyclin D1 expression was substantially down-regulated in BMSCs at both gene and protein levels. Simultaneously, BMSCs began to express nestin. No significant differences in cyclin D1 expression in BMSCs were detected between 12 and 24 hours post-treatment and the pretreatment level (P>0.05). Conclusion: Our results indicated that astragaloside A regulated cell cycle, prolonged G1/S phase transition, triggered BMSC differentiation, and finally induced BMSC differentiation into neuronal stem cells. The trophism of astragaloside A accelerated further growth of cell processes and the differentiation of cells into neurons.