Abstract: Ralstonia solanacearum is a worldwide soil born bacterium on tobacco, and its pathogenicity is affected by many factors but the key factors are still unclear. In this study, the effect of temperature, relative humidity, inoculum amount and pH on pathogenicity of R. solanacearum was systematically analyzed. The method of stab inoculation on detached leaves was used throughout the whole experiment. The results showed that the pathogenic amount of R. solanacearum was between 1.3 cfu and 1.3×10⁸ cfu, and the biggest scab in detached tobacco leaves was observed in the treatment of 1.3×10⁸ cfu. The pathogenic temperature of the bacterium was from 20℃ to 35℃, and the best range was from 30℃ to 35℃, showing decreased incubation period with the raise of temperature. Tobacco bacterial wilt could happen under the relative humidity range of 40% to 100%, thus humidity was not the key factor for this disease. The pathogenic pH value range was from 5.0 to 8.0, and the best value was around 6.0. In conclusion, temperature and pH value were the key factors for pathogenicity of R. solanacearum. These results provided scientific evidence for occurrence mechanism of tobacco bacterial wilt.