Objective To investigate the value of the absolute number of lymphocyte subpopulations as an early warning indicator for children with SMPP（severe mycoplasma pneumonia， SMPP） and to analyze the characteristics observed via bronchoscopy， thereby providing a valuable reference for the early diagnosis of SMPP. Methods This study included 102 children with Mycoplasma pneumoniae pneumonia （MPP）， comprising 54 cases of common MPP and 48 cases of SMPP. The lymphocyte subpopulations， clinical characteristics， and laboratory indicators were analyzed. Results There were statistically significant differences between the two groups in the absolute number levels of lymphocyte subpopulations CD3⁺CD19^-T， CD4⁺T， CD3^-CD19⁺B， CD3^-/CD16⁺CD56⁺NK cells （P < 0.05）. The absolute numbers of CD3⁺CD19^-T， CD4⁺T， CD3^-CD19⁺B， and CD3^-/CD16⁺CD56⁺NK cells showed negative correlations with serum ferritin， LDH， CRP， and D?D， respectively （P < 0.05）. Multifactorial logistic regression analysis identified the absolute numbers of CD3⁺CD19^-T， CD4⁺T， CD3^-CD19⁺B， and CD3^-/CD16⁺CD56⁺NK cells as independent risk factors for severe Mycoplasma pneumoniae pneumonia （SMPP）. ROC analysis demonstrated that the areas under the curve for diagnosing SMPP based on the absolute numbers of CD3⁺CD19^-T， CD4⁺T， CD3^-CD19⁺B， and CD3^-/CD16⁺CD56⁺NK cells were 0.711， 0.887， 0.856， and 0.860， respectively， with sensitivities of 47.4%， 80.8%， 82.1%， and 92.3%， and specificities of 89.7%， 87.2%， 75%， and 70.5%， respectively. The combined ROC curve of the four lymphocyte subsets had an area of 0.983， with a sensitivity of 97.4% and specificity of 92.3%. The proportions of bronchoscopy findings and microscopic examination of mucus plugs in the SMPP group were significantly higher than those in the ordinary MPP group （P < 0.05）. In the mucoid plug subgroup， the absolute numbers of CD3?CD19?T cells and CD4?T cells were significantly lower compared to the non?mucoid plug subgroup， while the percentage of CD8?T cells increased and the CD4?/CD8? ratio decreased （all P < 0.05）. Conclusions The absolute number of CD3⁺CD19^-T， CD4⁺T， CD3^-CD19⁺B， and CD3^-/CD16⁺CD56⁺NK cells in peripheral blood serves as a highly sensitive and specific predictor of small airway mucus plugging phenomenon （SMPP） and can thus be utilized as a potential biomarker for SMPP. Microscopic analysis under SMPP conditions reveals a high prevalence of mucus plugs， necessitating proactive bronchoscopic intervention. Furthermore， the significant imbalance in T cell subpopulations is strongly correlated with the formation of mucus plugs observed during bronchoscopy.