OBJECTIVE To explore the effect of hypoxia-supported umbilical cord mesenchymal stem cell (UC-MSC) on the expansion of cord blood mononuclear cell （MNC） in vitro. METHODS The isolated cord blood mononuclear cells were inoculated on the preestablished umbilical cord mesenchymal stem cell layer and cultured under hypoxic conditions (3％ O2) and the experimental groups were normoxia (MNCs were cultured under normoxic conditions), hypoxia (MNCs were cultured under hypoxic conditions), UC-MSC (MNCs were cultured with UC-MSC under normoxic conditions), and UC-MSC+hypoxia (MNCs were cultured with UC-MSC under hypoxic conditions). To further investigate the combinational effect of 3 factors of SCF+FL+TPO (SFT) on expansion of cord blood MNCs in vitro in hypoxia-supported UC-MSC culture system, the experiments were further divided into group A (MNCs were cultured with UC-MSC and SFT under normoxic conditions), group B (MNCs were cultured with UC-MSC under hypoxic conditions), group C (MNCs were cultured with UC-MSC and SFT under hypoxic conditions). The number of nucleated cells (TNC), CD34+ cell, CFU and CD34+CXCR4+, CD34+CD49d+, CD34+CD62L+ cells of each groups were detected at 0, 7, 10 and 14 days, respectively. RESULTS Compared with group hypoxia and UC-MSC, group UC-MSC+hypoxia effectively promoted the expansion of TNC, CD34+ cell and CFU, and upregulated the expression level of adhesion molecule and CxCR4 of the cord blood CD34+ cell（P<0.05）. After culturing for 14 days, compared with group A and group B, group C effectively promoted the expansion of cord blood MNC at different time points（P<0.05）, and the effect of group A was better than that of group B at 7 and 10 days（P<0.05）. CONCLUSIONS Hypoxia-supported UC-MSC efficiently promoted the expansion and expression of adhesion molecule and CXCR4 of cord blood CD34+ cell, and the effect of expansion could be enhanced when SFT 3 factors were added.