Objective To summarize and analyze the main clinical characteristics, feature and composition changes of gut microbiota in elderly patients with severe pneumonia, and to further explore the potential correlation between the gut characteristics and the etiology of severe pneumonia in elderly patients. Methods Patients with severe pneumonia admitted to the respiratory intensive care unit of a tertiary teaching hospital in Changsha were selected as the research subjects.Patients aged ≥65 years were assigned to the elderly severe pneumonia group, while those aged <65 years were assigned to the non-elderly severe pneumonia group. Based on clinical characteristics and pathogen detection of lower respiratory secretion, the elderly severe pneumonia group was further divided into a pulmonary bacterial infection group and a pulmonary fungal infection group. The pulmonary bacterial infection group was subdivided into Gram-positive bacteria group and Gram-negative bacteria group based on Gram-staining results. Clinical data of patients were collected, and fecal specimens within 24 hours after admission were obtained for 16S rRNA sequencing. Differences in gut microbiota characteristics between two groups of patients were compared, and the correlation between clinical characteristics of patients in the elderly severe pneumonia group and the abundance of differential microbiota was analyzed. Subsequently, the gut microbiota characteristics of elderly patients in severe pneumonia group infected by different pathogens were analyzed. Results Gut microbiota analysis showed no significant statistical differences in α- and β-diversity indicices between patients in the elderly and non-elderly severe pneumonia groups (both P>0.05). Linear discriminant analysis effect size (LEFSe) analysis indicated that, compared with patients in the non-elderly severe pneumonia group, the relative abundance of opportunistic pathogens, including Pseudomonadales, Moraxellaceae, and Acinetobacter, was significantly higher in patients in the elderly severe pneumonia group (all P<0.05). Some differential gut microbiota in two groups of patients were correlated with clinical indicators in patients in the elderly severe pneumonia group (all P<0.05). β-diversity analysis (principal coordinate analysis) combined with Anosim analysis revealed that in patients in elderly severe pneumonia group, there was significant differences in gut colony structures between patients in the bacterial and fungal infection groups (R=0.149, P=0.02). Compared with the fungal infection group, patients in bacterial infection group showed a significantly reduced abundance of probiotics, including Verrucomicrobiales and Collinsella, and opportunistic pathogens such as Akkermansia (all P<0.05). Conclusion Elderly patients with severe pneumonia have a dysregulated gut microbiota with significantly increased abundance of pathogenic bacteria compared with non-elderly patients. Differential gut microbiota of two groups of patients are correlated with some infection-related and organ function indicators in elderly patients with severe pneumonia. Compared with elderly patients with severe fungal pneumonia, those with severe bacterial pneumonia have significant differences in gut colony structures and a notably reduction in probiotics abundance.