Fuente: WIPO "tobacco agriculture" The invention discloses a complex microbial inoculant for preventing and treating tobacco mildew and rot, a fermentation method and an application method of the complex microbial inoculant. The complex microbial inoculant at least comprises a biocontrol bacterium BA08, a biocontrol bacterium BA20 and a biocontrol bacterium BSRS-1, the biocontrol bacterium BA08 is a strain of bacillus subtilis, and the preservation number of the biocontrol bacterium BA08 is CCTCC No: M 2020549; the biocontrol bacterium BA20 is a strain of bacillus subtilis, and the preservation number of the biocontrol bacterium BA20 is CCTCC No: M 2020548; the biocontrol bacterium BSRS-1 is a strain of enterobacter mori, and the preservation number of the biocontrol bacterium BSRS-1 is CCTCC (China Center for Type Culture Collection) No: M2020550. The complex microbial inoculant is good in stability, not only is an efficient biocontrol bacterium for tobacco mildew and rot in the baking period, but also has a good control effect on mildew and rot in the cigar fermentation process. Therefore, the complex microbial inoculant provided by the invention can exert a relatively good prevention and treatment effect on the tobacco mildew and rot disease.