The pot experiment with the soil containing pathogens of Fusarium oxysporum f. sp. was conducted to study the effects of Bacillus megaterium B213,Bacillus polymyxa B207,and Bacillus subtilis B204 alone or in combination on cucumber fusarium wilt and its mechanism. The results showed that the three microbial agents alone or in combination effectively inhibited the occurrence of cucumber fusarium wilt and significantly increased the biomass of cucumber. The effect of the two microbial agents in combination was better than that of single microbial agent. The effect of the three microbial agents in combination was better than that of the two microbial agents. Compared with the control group,the incidence of cucumber fusarium wilt in the three-agent combination group when cucumber seedlings were transplanted 35 days ago reduced by 79.99%; the dry weight of cucumber plants increased by 6.92 times; the activities of plant resistance enzymes including acid lyase,polyphenol oxidase,and peroxidase increased by 60.02%,28.90%,and 108.60% respectively; the number of rhizosphere soil Fusarium oxysporum decreased by 2 orders of magnitude. The effect of disease prevention and growth promoting may be related to the antagonism on the pathogenic bacteria,the ability to dissolve phosphorus,and the secretory ability of indoleacetic acid,protease,chitinase and ferrophilin of these three Bacillus species.