157 bacteria strains were isolated from tobacco field soil between roots and around root system in Shandong Province. 7 strains of antagonistic bacterium against Alternaria alternata were screened. Among them, strain B102 had the strongest antagonistic effect with an inhibiting zone up to 13.0mm in width. The antagonism of B102 ferment filtrate and metabolic products against pathogens were examined with the methods of asynchronous cultivation, duel cultivation and carrier cultivation. The results showed that the antagonistic activity of strain B102 mainly relied on the resistance to high temperature of its metabolic product, which inhibited the growth of pathogen mycelium, germination of spores, break of mycelium cell and seepage of endogenous substances, and made pathogen lose its infection activity. After treated in conditions of various pH values and high temperatures, B102 metabolic product still presented relatively high antagonistic activity, it indicated that strain B102 was highly adaptable to different environments.