Abstract: 【Objective】 This study is to investigate the antagonistic ability and mechanism of Rahnella aquatilis JZ-GX1 strain against pathogens causing poplar canker to improve the biocontrol of polar canker. 【Method】 The control efficiency of JZ-GX1 strain was studied by antagonistic testing in dural culture, determining of the extracellular metabolites inhibitory rate, observation under SEM(scanning electron microscopy)and inoculation via root pouring and stem coating on one-year-old cuttings of ‘NL 895' polar(Populus×euramericana CL‘NL895'). 【Results】The results showed that R. aquatilis JZ-GX1 strain had significant antagonistic ability to C. chrysosperma and Phomopsis sp., but not to Fusicoccum aesculi. The extracellular metabolites of JZ-GX1 strain showed better antagonistic ability to Cytospora chrysosperma with 77.87% inhibitory rate. The occurrence rate of the disease was lower than the control under root pouring and stem coating treatment, and the control efficiency of JZ-GX1 strain against C. chrysosperma was 84%. The fermented suspension of JZ-GX1 strain can change the morphology of the hyphae of C. chrysosperma to be more thinned, twisted and more branches under SEM. JZ-GX1 strain also can produce siderophore, cellulase and phosphatase, which significantly affect the cell membrane permeability, the concentration of soluble sugar and MDA. The concentration of soluble sugar(1 584.75 μmol/L )decreased comparing to the control after treatment and the cell membrane permeability(with relative conductance of 77.08%)and MDA content increased MDA concentration of 0.408 μmol/L). JZ-GX1 strain can colonize in poplar. 【Conclusion】 Rahnella aquatilis JZ-GX1 strain had a potential to be used as bio-control agent against poplar canker.