Objective To observe the promoting effect of hypoxia on proliferation of human bone marrow mesenchymal stem cells (BMMSCs) and maintaining their potential of multi-directional differentiation in vitro. Methods BMMSCs were isolated from bone marrow blood samples of patients with bone fracture, and cultured under hypoxic (group A) or normoxic (group B) conditions. The morphology, proliferation and osteogenic and adipogenic potential of the BMMSCs were observed. Results BMMSCs in the group A showed a long spindle shape and a fish shoal-like distribution, and were well-grown, while the morphology of cells in the group B appeared polygonal or flat. The quantity and growth rate of BMMSCs in the group A were increased compared with the group B (P< 0.05), with an osteogenic and adipogenic potential. Conclusions Hypoxia can promote the proliferation of BMMSCs in vitro and maintain their multi-directional differentiation potential.