Abstract: The inhibition activity of the strain SH7 to tobacco wilt（Ralstonia solanacearum）, isolated from the rhizopheres of tobacco plants, was studied. Disease control with SH7 bacterium-free ferment was carried out. We extracted its antimicrobial substance, too. The results indicated that the strain SH7 was strongly antagonistic to Ralstonia solanacearum. Compared to those of CK in 29d after treatment, the control effects of SH7 bacterium-free ferment, which included two treatments (watering roots with SH7 bacterium-free ferment and with the 30 min mixed suspension of SH7 bacterium-free ferment and Ralstonia solanacearum) were 65.9% and 53.2%, respectively. It implied that the excretion of SH7 is a very important antagonistism. This antimicrobial substance was purified by ammonium sulfate with 70% saturation, and we pre-concluded that it belongs to a kind of protenlike substances.