The invention discloses bacillus siamensis X. The collection number of the bacillus siamensis X in the China Center for Type Culture Collection is CCTCC NO: M 2017008; a strain of the bacillus siamensis X is applied to the prevention and the treatment of the powdery mildew of the plant, compared with the existing chemical preventing and treating agent, the bacillus siamensis X is obvious in prevention and treatment effects, can be used for effectively controlling and relieving the state of an illness, can be used for making a disease plant recuperate health, further, is stable in the prevention and treatment effects, makes the recrudescence difficult, is safe, nontoxic, harmless and environment-friendly, and can be used for effectively avoiding pesticide-residue and environmental-pollution problems brought by the prevention and the treatment of a chemical pesticide; further, a preparation method of a product provided by the invention is simple, and is easy to operate; the bacillus siamensis X is suitable for industrialized production and market popularization and application.