Objective To investigate the relationship between 9 immunoinflammatory indicators derived from complete blood count and the severity of Mycoplasma pneumoniae pneumonia (MPP) in children of different ages. Methods Totally 2 132 children with MPP who were hospitalized in the Department of Pediatrics of The First Affiliated Hospital of Naval Medical University from Jul. 1, 2023, to Dec. 31, 2024 were enrolled, and were assigned to severe MPP (SMPP) or non-severe MPP (NSMPP) groups. According to age and gender 1∶1 matching, the children were assigned to 2 subgroups according to age (1-6 years old and ＞6-16 years old). The basic data, laboratory examination and immunoinflammatory indicators from complete blood count of each group were collected and compared. The influencing factors of SMPP were analyzed by univariate and multivariate Cox proportional hazards regression models. Receiver operating characteristic curves were used to analyze the predictive value of indicators that showed statistically significant differences for SMPP. Results There were 220 patients with SMPP, accounting for 10.3% of MPP. In children aged 1-6 years, compared with the NSMPP group, the SMPP group had a longer hospital stay, higher platelet (PLT) count, platelet-to-lymphocyte ratio (PLR), neutrophil-to-lymphocyte ratio, derived neutrophil-to-lymphocyte ratio and systemic immune-inflammation index (all P＜0.05). PLR was an independent risk factor for SMPP (odds ratio＝1.010, 95% confidence interval ［CI］1.003-1.018, P＝0.007). The area under curve predicted by PLR for SMPP was 0.635 (95%CI 0.560-0.711, P＜0.001), the best cut-off value was 125.04, and the corresponding sensitivity and specificity were 57.7% and 70.2%, respectively. All the children were assigned to low PLR group or high PLR group using the best cut-off value as the boundary, and the severe disease rate in the high PLR group was significantly higher than that in the low PLR group (65.9%［60/91］ vs 37.6%［44/117］, P＜0.001). All the children were assigned to Q1-Q4 groups by quartile, and the severe disease rate of the Q4 group (71.2%, 37/52) was significantly higher than that of the Q1-Q3 group (all P＜0.05). In children aged ＞6-16 years, compared with the NSMPP group, the PLT and PLR in the SMPP group were higher (both P＜0.05), but neither was an independent risk factor. All the children were assigned to low PLR group or high PLR group using the best cut-off value (137.03) as the boundary, and the severe disease rate in the high PLR group was significantly higher than that in the low PLR group (57.0%［77/135］ vs 40.2%［39/97］, P＝0.011). All the children were assigned to Q1-Q4 groups by quartile, and the severe disease rate of the Q4 group (65.5%, 38/58) was significantly higher than that of the Q1-Q3 group (all P＜0.05). Conclusion The immunoinflammatory indicators derived from complete blood count, especially PLR, have certain application value in predicting the severity of MPP children in different ages.