Objective:To explore the structure and diversity of intestinal flora in patients with community acquired pneumonia (CAP) before and after treatment with cefotaxime combined with levofloxacin.Methods:From October to December 2018, 6 patients with CAP in the Department of Infection, Zhejiang Provincial Hospital of Tongde, were treated with cefotaxime injection 2.0 g (once/8 h) combined with 0.5 g levofloxacin injection (once a day). A total of 12 fecal samples were collected before and after 7 days of treatment. The stool samples before and after treatment were analyzed by 16S rRNA sequencing.Results:⑴ The structure of intestinal flora before and after treatment : at the phylum level: Firmicutes 59.2% vs 40.8%, Proteobacteria 18.6% vs 35.5%, Bacteroidetes 14.8% vs 20.8%, Actinobacteria 5.6% vs 1.2%; At the family level: Ruminococcaceae 34.5% vs 13.0%, Lachnospiraceae 15.9% vs 9.7%, Veillonellaceae 1.8% vs 3.3%, Lactobacillaceae 0.3% vs 8.0%, Streptococcaceae 2.9% vs 1.1%, Enterococcaceae 0.02% vs 5.2%, Enterobacteriaceae 16.4% vs 34.6%, Bacteroidaceae 13.3% vs 16.8%, Porphyromonadaceae 0.3% vs 3.4%, Adlercreutzia 4.4% vs 0.5%. There was no significant difference in the composition and structure of intestinal flora before and after treatment ( P>0.05). ⑵ The diversity of intestinal flora before and after treatment: operational taxonomic units (OTU) mean (150.5±59.0) vs (93.2±34.1), t=2.72, P=0.04; Chao1 index (169.25±49.61) vs (117.92±35.06), t=3.22, P=0.02; shannon index (3.61±0.83) vs (2.31±0.73), t=4.54, P=0.01; simpson index (0.80±0.10) vs (0.61±0.20), t=2.76, P=0.04. There were significant differences in the diversity of intestinal flora before and after treatment ( P<0.05). ⑶ There was significant difference in desulfovibrio between the two groups before and after treatment (LDA=2.03, P=0.02). Conclusions:After intravenous infusion of cefotaxime combined with levofloxacin for one week , the diversity of intestinal flora was significantly reduced after treatment. Desulfovibrio was the flora with statistical differences between before and after treatment.