Abstract: The present disclosure discloses a strain of Lactobacillus fermentum capable of preventing and/or treating periodontitis and an application thereof, belonging to the field of microbiology technology. The present disclosure has screened to obtain a strain of L. fermentum CCFM1139. The L. fermentum CCFM1139 can alleviate periodontitis by inhibiting formation of mixed bacterial biofilms of Porphyromonas gingivalis, Prevotella intermedia and Fusobacterium nucleatum, thereby reducing the amount of biofilm by 52.45%, and can reduce the content of TNF-? from 131.37 pg/mL to 83.31 pg/mL and IL-8 from 147.70 pg/mL to 121.12 pg/mL in a periodontitis cell model, reduce colonization of the P. gingivalis and the F. nucleatum in the oral cavity by 1-2 orders of magnitude, and reduce the amount of the alveolar bone resorption from 1838.0 ?m to 805.7 ?m.