Object: To investigate the effect of photodynamic therapy with TBO as photosensitizer on inhibition of cariogenic bacteria in rat biofilm.Method：A model of dental plaque biofilm was established by using streptococcus mutans, streptococcus haematococcus, lactobacillus acidophilus and actinomyces glutinous as experimental strains. The experiment was divided into five groups. Forty rats were randomly divided into five groups. There were 8 rats in each group. Group A was the negative treatment group treated with normal saline; Group B was the positive control group treated chlorhexidine; Group C was treated with laser radiation only. The parameter was a wavelength of 630 nm, an output intensity of 105mW/cm2, irradiation time of 9 min; Group D was treated with photosensitizer only whose concentration was 100mg/L and incubated in dark for 5min; Group E was named TBO-PDT group. TBO-PDT group was treated with 100 mg/L TBO , incubated in dark for 5min and irradiated with diode laser which parameter was wavelength 630nm and output intensity 105mw/cm2.The oral cavity of each rat was divided into 4 regions. The last two molars in each area were selected as experimental objects. 8 sample teeth were selected for each rat.The activity of dental plaque biofilm was observed by plate colony counting experiment. The effect of PDT on oral soft tissue was observed by histopathological section. Scanning electron microscopy was used to observe the changes of tooth surface morphology before and after PDT treatment.Result: Compared with Group A and Group B, the number (CFU/mL) of viable caries causing bacteria in the dental plaque of Group E was significantly reduced (P<0.05). The bacteriostasis rate was 87.02%. Group C and Group D have no obvious bacteriostatic effect. Scanning electron microscopy showed that the molars of TBO-PDT group had smooth surface and superficial demineralization pore. Pathological section showed no obvious periodontal tissue damage in TBO-PDT group.Conclusion: The experimental results showed that photodynamic therapy had obvious anti-caries effects and had no obvious damage to the dental hard tissue and periodontal tissue.