Abstract: To investigate the control effect of Bacillus amyloliquefaciens YN201728 on tobacco powdery mildew, and to clarify the influences of YN201728 on the activities of defense enzymes (peroxidase POD, phenylalnine ammonialyase PAL and polyphenol oxidase PPO) in tobacco leaves infected by pathogen of tobacco powdery mildew (Erysiphe cichoracearum), pot experiments were conducted to analyze the control effects of YN201728 fermentation broth, fermentation filtrate, bacterial cell precipitation resuspension, bacterial cell crushing liquid on tobacco powdery mildew. The water agar slide method was used to investigate the inhibitory effect of the fermentation filtrate on spore germination of Erysiphe cichoracearum, and the changes in the activities of POD, PAL and PPO in tobacco leaves inoculated with YN201728 fermentation broth and Erysiphe cichoracearum were analyzed through pot experiments. The results showed that there was no significant difference in the relative control efficacies on tobacco powdery mildew between fermentation broth treatment and the fermentation filtrate treatment under pot conditions, and the control efficacies reached 61.64% and 56.95% respectively after 7-day treatments, and the highest relative control efficacies of the treatments of bacterial cell precipitation resuspension and bacterial cell crushing liquid were 18.70% and 10.80%, respectively. Tested fermentation filtrate had a significant inhibitory effect on spore germination, and the inhibition rate of the fermentation filtrate with the content (volume fraction) of 20.0% on spore germination of Erysiphe cichoracearum was 92.07%. When tobacco was infected with Erysiphe cichoracearum, the activities of POD and PAL in tobacco leaves treated with the fermentation broth were not significantly higher than those treated with aseptic water, the strain did not improve the resistance to tobacco powdery mildew by inducing increases in the activities of POD and PAL defense enzyme.