Objective:To evaluate the differential expression of blood routine in different types of infection and the diagnostic value of C-reactive protein (CRP), procalcitonin (PT), ferritin (SF) and lactate dehydrogenase (LDH) in bacterial and mycoplasma pneumonia and their early warning value in severe cases.Method:A total of 627 patients, including 176 cases of bacterial pneumonia, 275 cases of mycoplasma pneumonia, 176 cases of viral infection and 180 cases of normal control were collected from May 2018 to December 2019 in children′s Hospital Affiliated to Capital Institute of Pediatrics. The mycoplasma pneumonia group was divided into mild group (151 cases) and severe group (124 cases) according to the results of lavage fluid RNA-examination. All patients received completed blood routine test at the first day of admission, patients in bacteria group and Mycoplasma group received the examination of four inflammatory indicators. The Kruskal-Wallis test was used to analyze the differences in blood routine results between different infection groups, and the differences of inflammatory indexes between bacterial group and Mycoplasma mild and severe group. The receiver operating characteristic (ROC)-curve method was used to analyze the predictive value of inflammatory indexes between different infection groups.Results:There were significant differences in leukocyte count, neutrophil, lymphocyte and monocyte percentage between bacterial pneumonia, mycoplasma pneumonia, viral infection and normal control group ( P<0.05). The differences of four inflammatory indexes in bacterial group, mild Mycoplasma group and severe group were statistically significant ( P<0.05). The rest of the index (CRP, PCT, LDH, SF and white blood cell count) were P<0.05 (CRP: area under curve [AUC] 0.799; PCT: AUC 0.579; LDH: AUC 0.651; SF: AUC 0.854), in mild and severe mycoplasma group, except WBC, by ROC-curves analysis. The AUC value of the area under the curve of CRP and SF is high, and the sensitivity and specificity at the diagnostic critical point are high, which has great diagnostic value (CRP: diagnostic critical point 12.55 mg/L, sensitivity 0.719, specificity 0.755; SF: diagnostic critical point 176.02 μg/L, sensitivity 0.765, specificity 0.960). ROC curve results also showed that of PCT, White blood cell and neutrophil percentage had the diagnostic value in bacterial infection and mycoplasma infection, P<0.05 (PCT: AUC 0.658; leukocyte: AUC 0.804; neutrophil: AUC 0.630). Leukocyte count is the best differential index (diagnostic critical point 9.585×10 9/L, sensitivity 0.778, specificity 0.698), PCT has higher sensitivity at the diagnostic critical point of 0.55 μg/L, but the specificity is slightly lower (diagnostic critical point of 0.55 μg/L, sensitivity 0.862, specificity 0.366). Conclusions:PCT and leukocyte count can be used as the preferred inflammatory indexes to distinguish bacterial and mycoplasma infection. CRP, LDH, PCT and SF can be used as early warning indexes to evaluate severe mycoplasma infection.