Objective To analyze the etiology of bronchoalveolar lavage fluid and prognosis of children with severe pneumonia. Methods In this study, 40 children with severe pneumonia who were hospitalized in the pediatrics department of our hospital and underwent alveolar lavage treatment from March 2019 to December 2020 were selected as the research objects. Through the detection of pathogens such as bacteria, fungi, Mycoplasma pneumoniae and T lymphocyte subsets of these children in the acute phase of infection, we can understand the pathogens and drug resistance of children with severe pneumonia in Taishan area and the relationship among drug resistance, T lymphocyte subsets and disease severity and prognosis assessment. Results In the analysis of the BALF pathogenic test results, there were 27 cases with Mycoplasma pneumoniae, 5 cases with Mycoplasma pneumoniae+Streptococcus pneumoniae, 2 cases with Mycoplasma pneumoniae+Staphylococcus intermedius, 1 case with Mycoplasma pneumoniae+Enterococcus casseliflavus, 2 cases with Mycoplasma pneumoniae+Pseudomonas maltophilia and 3 cases were pathogenic bacteria negative. The test results of serum T cell subsets of these cases showed that most of the cases had different degrees of decline in the levels of CD3⁺, CD4⁺, CD8⁺ and CD4⁺/CD8⁺. Among them, CD3⁺ levels decreased in 6 cases, CD4⁺ levels decreased in 16 cases, CD8⁺ levels decreased in 17 cases, CD4⁺ and CD8⁺ levels decreased in 14 cases, and CD3⁺, CD4⁺, and CD8⁺ levels decreased in 4 cases; total cell number of BALF was (3 673.1±377.9)×10⁶/L, the proportion of macrophages was (23.6±17.6)%, lymphocytes had (22.1±16.2)%, and neutrophils had (46.5±24.8)%. Conclusions Pathogenic analysis showed that the main pathogen of BALF in children with severe pneumonia is Mycoplasma pneumoniae, and the detection of serum T cell subsets mostly showed a decrease in CD4⁺ and CD8⁺ levels.