Abstract: Objective　To analyse the relationship between immunosuppression and severity of sepsis in children, thus to explore the role of immunity in the progression of sepsis and provide a reference for the treatment of sepsis in children. Methods　In this retrospective study, 375 children with sepsis were included. Blood specimens were collected for the analysis of lymphocyte subsets and humoral immunity in 24 hours after admission into ICU. Univariate analysis was performed between immunity indices and organ dysfunction. Logistic stepwise regression was used to screen out the risk factors of multiple organ dysfunction (MKDS) and mortality. Results　Sepsis children were divided into three groups, no organ dysfunction, single organ dysfunction and multiple organ dysfunction. NK cell percentage, CD4/CD8 and C3 were different among three groups, and significantly lower in MODS children. Multivariate analysis showed decreased CD4/CD8 (OR=0.66, 95%CI:0.51-0.85). C3 level (OR=0.20, 95%CI:0.10-0.42) were associated with MODS. CNS infection (OR=2.54, 95%CI:1.35-4.77) and pneumonia infection (OR=2.29, 95%CI:1.36-3.83) were also risk factors of MODS. Depletion of C3 (OR=0.26, 95%CI:0.09-0.76) and pneumonia infection (OR=2.61, 95%CI:1.12-6.09) were risk factors of mortality. Conclusions　CD4/CD8 and complement C3 were significantly decreased in sepsis children with MODS, immunosuppression may play an important role in the development of MODS in children with sepsis. Baseline CD4/CD8 and C3 level could be potential prognostic markers of sepsis.