The purpose of this study is to explore the role of different T cell subgroups in the pathogenesis of sepsis in children. Flow cytometry was used to detect the changes in the activation status and the number of T cell subgroups in the peripheral blood of children with sepsis; healthy children were selected as the control group. Compared with healthy children, the number of CD4+ T cells in the peripheral blood of children with sepsis did not change significantly (Z = 1.945, P = 0.052); though the ratio decreased and the median level dropped from 34.6% to 30.7% (Z = 2.257, P = 0.024). However, the number of CD8+ T cells in the blood of children with sepsis increased, and the median level also increased from 0.2 × 10/L to 0.4 × 10/L (Z = -2.404, P = 0.016). In addition, CD3+CD8+HLA-DR + cell level significantly increased, and the median level increased from 4.2% to 24.3% (Z = -5.370, P = 0.000). There was a large heterogeneity in the hospitalization time of sepsis in clinical patients. Compared to patients with a mean hospital stay of 6 days, patients with a median hospital stay of 13 days had a lower CD3+CD4+CD25 + cells percentage, while the percentage of CD3+CD8+HLA-DR+ was higher, resulting in a more apparent increase of CD3+ CD8+HLA-DR+/CD3+CD4+CD25+. Therefore, the failure of CD4+ T cell activation and proliferation, and the excessive activation and proliferation of CD8+ T cells play an important role in the pathogenesis of sepsis. The increase of CD3+CD8+HLA-DR+/CD3+CD4+CD25 + ratio was associated with the extended course of sepsis.