In China, the saline-alkali soil covers an area of approximately 99.13 million hm², of which the severe saline-alkali soil with pH value above 9 and salinity greater than 0.6% represents a growth rate of 1.4% per year. Improving plant rhizosphere environment by utilization of nitrogen-fixing microorganisms to increase crop yield is an important method for saline-alkali soil amelioration.[Objective] Candidate strains were selected for the amelioration of extreme saline-alkali soils by isolating the autogenous azotobacter from the soil sample collected from the coral reefs in Sansha City, Hainan Province. [Methods] The strains were identified through morphological observation, physiological and biochemical characteristics analysis and 16S rRNA sequencing, followed by the analysis of their nitrogen-fixing, salt-tolerant and growth-promoting characteristics, and verification of their effects on the main agronomic traits of corn. [Results] One strain of extremely salt-tolerant nitrogen-fixing bacteria named DJ-1 was obtained. The colony of the strain is round in shape,The individual bacterium is rod-shaped,sizing (0.3-0.5) μm×(0.5-1.3) μm. Negative in Gram Staining, the strain is highly homologous with the 16S rRNA sequence of Agrobacterium tumefaciens, hence it is determined to be Agrobacterium tumefaciens. Under medium culture condition, DJ-1 presents normal growth at pH value of 9 with NaCl concentration of 1%-4%, and tolerance at pH value of 12 and NaCl concentration of 8%. The nitrogenase gene nifH was cloned from DJ-1. It has been indicated in the pot experiment results that the growth of corn was significantly accelerated by DJ-1. [Conclusion] The strain DJ-1 has the tolerance to extreme saline-alkali conditions, coupled with high nitrogen fixation and growth-promoting capabilities. It may be a potential candidate strain of functional bacteria for poor saline-alkali farmland amelioration.