To explore the application value of T lymphocyte subsets combined with procalcitonin (PCT), C-reactive protein (CRP), neutrophil to lymphocyte ratio (NLR) and white blood cell count (WBC) in the auxiliary diagnosis and prognosis evaluation of sepsis. In a retrospective study, seventy-two patients with sepsis diagnosed and treated in Tianjin First Central Hospital from June 2018 to April 2021 were selected as the research objects, and included in the sepsis group were 46 males and 26 females, aged 68 (57.3, 80.3) years. In addition, 111 patients with local infection admitted to hospital during the same period were included in the local infection group, including 62 males and 49 females, aged 68 (51, 77) years. Sepsis patients were divided into survival group (43 cases) and death group (29 cases) according to the 28-day outcome. CD3, CD4, CD8, CD4/CD8 ratio were detected by flow cytometry within 24 h after admission, PCT was detected by ELISA, CRP was detected by immunoturbidimetry, blood routine examination, blood lactic acid (Lac) and oxygen partial pressure (PO) were detected by instrumental method. Multivariate Logistic regression analysis was used to evaluate the correlation between each indicator and sepsis, and receiver operating characteristic curve (ROC) was drawn to evaluate the diagnostic value of each indicator for sepsis. Multivariate Logistic regression analysis and Kaplan Meier survival analysis were used to evaluate the prognostic value of each index for patients with sepsis. Peripheral blood CD3, CD4, CD8, CD4/CD8 ratio and PLT in sepsis group were significantly lower than those in local infection group(=-8.184,<0.001;=-7.210,<0.001;=-5.936,<0.001;=-2.700,=0.007;=-6.381,<0.001); PCT, CRP, NLR and Lac levels were significantly higher than those in local infection group(=-8.262,<0.001;=-3.094,=0.002;=-9.004,<0.001;=-4.770,<0.001). Multivariate Logistic regression model showed that PCT, NLR, CD3, CD8, CD4/CD8 were independent risk factors for sepsis. According to ROC curve analysis, AUC of sepsis patients diagnosed by each indicator were 0.862, 0.894, 0.858, 0.760 and 0.618, respectively. The cut-off values were 3.075 ng/ml, 10.715, 44.935×10/L, 27.463×10/L and 0.750, respectively. The NLR sensitivity was 80.6%, and the CD3 specificity was 94.6%. The AUC of combined detection of PCT and NLR was 0.947, sensitivity was 87.5% and specificity was 91.9%. The combined detection AUC of PCT, NLR, CD3, CD4/CD8 was 0.958, the sensitivity and specificity were 90.3% and 91.0% respectively(<0.001). PCT and Lac in death group were significantly higher than those in survival group(=-2.302,=0.021;=-3.095,=0.002);Peripheral blood CD4/CD8 levels were significantly lower than those in survival group(=-3.691,<0.001),Multivariate Logistic regression model showed that CD4/CD8 ratio was an independent risk factor for 28 d mortality in patients with sepsis (<0.001). The ROC curve showed that the AUC was 0.758, and the Youden index reached the maximum when the cut-off value was 1.27, the sensitivity and specificity were 79.3% and 60.5%, respectively. Compared with patients with CD4/CD8 ≥1.27, 28-day mortality was significantly increased in patients with CD4/CD8<1.27 (=0.032). The combined detection of PCT, NLR, CD3 and CD4/CD8 can improve the auxiliary diagnostic efficiency of sepsis, and the ratio of CD4/CD8 in peripheral blood may have certain predictive value for the prognosis of sepsis.