The invention discloses an application of 
lactobacillus paracasei L9 to prevention or treatment 
oral disease and regulation of 
oral cavity flora. L9 can be used for preparing a medical composition anda food for preventing or treating dental caries. Research proves that L9 can notably relieve generation degree of 
occlusal surface E-level dental caries, after the LP is taken, the structure of the 
oral cavity flora is changed notably, particularly 
actinobacteria is notably increased, the 
actinobacteria of L9 group is obviously reduced, 
klebsiella, aerococcus, 
Gemella, 
corynebacterium and 
pseudomonas aeruginosa on the classification level are notably reduced, and the 
oral disease risk is reduced. When L9 is taken, the 
saliva species richness is notably reduced, the relative abundance of oraldisease relevant fungi of 
streptococcus salivarius, actinomycetaceae, 
peptostreptococcus and the like is notably reduced, and the L9 can have relieving effects on periodonitis diseases. The 
dental plaque flora species diversity is notably reduced, the relative abundance of leptothrix, fusobacteriaceaeand 
flavobacterium is notably reduced, and the L9 can have relieving effects on oral diseases of 
periodontal disease, halitosis, dental caries and the like.