Abstract: In recent years, with the development of research on chitosan, its unusual antibacterial activity attracted wide attention of scholars. In this study, in order to obtain the antibacterial effect of chitosan on eucalyptus Ralstonia bacteria and its antagonistic bacteria, the inhibition effect of different molecular weight of chitosan on 302- Paenibacillus polymyxa, 199- Bacillus cereus, 76- eggplant Ralstonia Lal coli and RSeucalyptus Ralstonia bacteria were researched. The method of disc agar diffusion was used to study antibacterial activity of different molecular weight of chitosan. The method of serial dilution was used to determine the minimal inhibitory concentration (MIC) of different molecular weight of chitosan. At the same time, the antibacterial effect of the molecular weight of 2200 chitosan under different pH value was also explored. The results showed that the inhibition effect of different molecular weights of chitosan on 76-eggplant Ralstonia Lal and RS-coli bacterial wilt pathogen was significant; especially the molecular weight of 2200 and 50000 chitosan has the best inhibition effect. With the increasing of chitosan concentration, the inhibition effect on the two bacteria above gradually increased, the Mic (minimum inhibitory concentration) was above 0.05%. The inhibition effect of different molecular weight chitosan on 302-Paenibacillus polymyxa and 199-Bacillus cereus was weaker comparing to 76-eggplant Ralstonia Lal and RS-coli bacterial wilt pathogen, the Mic was 0.1%. The best antibacterial effect was obtained on pH 5.5-6.5. So far, the antibacterial effect of different molecular weight of chitosan on the bacterial strain above had not been reported.