Abstract: To further reveal the control mechanism of avirulent strains of Ralstonia solanacearum against tobacco bacterial wilt, biocontrol trials were conducted in a greenhouse and field plots. The colonization of drug-resistant mutant strains in tobacco plants was determined via drug resistance marker, and the changes of activities of enzymes PAL, POD, PPO in the treated tobacco plants were analyzed. The results showed that the relative control efficacy of avirulent strain Tbw1-7-3 against tobacco bacterial wilt was up to 89.4% in greenhouse. At the early stage(10-20 days)after disease occurrence, the control efficacy of Tbw1-7-3 was comparable to that of streptomycin in fields, its drug-resistant mutant strain Tbw1-7-3k could colonize on the surface of tobacco roots and in tobacco plants for a short duration. Meanwhile,Tbw1-7-3 promoted the activities of PAL, POD, PPO and the accumulation of pathogenesis-related protein(PRP) in hosts. Avirulent strains possessed better control efficacy against tobacco bacterial wilt,especially strain Tbw1-7-3. The field experiments also indicated that Tbw1-7-3 had good control efficacy at the early stage of disease occurrence. The antagonistic mechanism of avirulent strains against Ralstonia solanacearum might be the results of both position occupying and resistance inducing.