Abstract: Objective To investigate the mechanisms of immunosuppression in patients with multiple trauma. Methods From January 2014 to January 2015, 55 patients with multiple trauma were enrolled as research group, and 30 healthy individuals as control group. One week after hospitalization, the blood sample was collected and stored for analysis. The serum level of C3a, C4a, C5a and C5b-9 (the active products of complement system) of patients in control group and research group were measured by rate scattering immune turbidimetry, the amount of mesenchymal stem cells and Tregs in periphery blood were detected by flow cytometry, and the serum IFN-γ, TGF-β and IL-10 level were tested by ELISA. Results The C3a, C4a and C5b-9 levels of the research group were significantly lower than those of control group (0.65±0.16) g/L vs (1.25±0.31) g/L, (0.18±0.06) g/L vs (0.32±0.18) g/L, (10.87±1.78) ng/L vs (26.38±2.28) ng/L, P＜ 0.05, while the C5a level in serum in research group was significantly higher than that of control group (185.56±3.45)μg/mL vs (131.58±12.15)μg/mL, P＜ 0.05. The results of flow cytometry indicated that the amount of MSC and Tregs of the research group were higher than those of the control group (3.98±0.25)% vs (2.58±0.12)%, (4.46±0.35)% vs (3.12±0.23)%, P＜ 0.05. The levels of IFN-γ, TGF-β and IL-10 in research group were significantly higher than those of control group (15.27±0.75) ng/ml vs (8.74±0.51) ng/ml, (10.08±0.65) ng/ml vs (5.29±0.47) ng/ml, (12.45±0.38) ng/ml vs (7.96±0.52) ng/ml, P＜ 0.05. Conclusion Patients with multiple trauma has a low serum level of complement gystem active products such as C3a, C4a and C5b-9. The low activity of complement system results in a state of immunosuppression, the periphery blood of the patients contains large amount of immunosuppressive cells such as MSC and Tregs, and the serum level of immuno-suppressors such as IFN-γ, TGF-β and IL-10 increases obviously leading to obvious immune suppression after trauma.