Abstract: In order to address the problem of unstable antibacterial activity of single biocontrol bacteria, a composite strains with high antagonistic activity against Phytophthora parasitica var. nicotianae was screened from six biocontrol bacteria by compatible test, antagonist test and pot test. The taxonomic status of each strain was identified with a phylogenetic analysis of the 16S rRNA and gyrA genes of the strains, and then the optimal fermentation medium for each strain was screened with a single factor test. The co-application of GY1, GY10 and GY12 after fermentation showed the best control effect on tobacco black shank, with the plate inhibition rate reached 86.9% and he potted control effect reached 84.21%, which increased by 17.58%, 26.74% and 35.9% compared with single strains of GY1, GY10 and GY12, respectively. Molecular identification showed that the three strains were Bacillus velezensis, Bacillus amyloliquefaciens and Bacillus subtilis, respectively. The optimal fermentation mediums of GY1, GY10, and GY12 were screened, where the biomass of GY1, GY10 and GY12 increased by 75.12%, 92.31%, and 194.55%, respectively, compared with the basic medium after 24 hours of fermentation.