In order to explore the improvement effects of Sesbania cannabina returning on saline soils in tidal flat under different nitrogen application treatments, a field plot experiment was conducted to study the effects of returning green manure Sesbania cannabina to the field on the contents of soil carbon, nitrogen, pH and total water-soluble salt and bacterial community structures under different nitrogen application treatments (CK, SN1, SN2, SN3, SN4 corresponding to nitrogen application rates of 0, 90, 135, 180, and 225 kg/hm², respectively). The results showed that the biomass, carbon and nitrogen accumulation of Sesbania cannabina were the highest under SN3 which were 41 882, 3 756and 101.5 kg/hm², respectively. After returning green manure to the field, soil organic carbon and total nitrogen contents of SN2 were the highest, which were 6.51 g/kg and 0.637 g/kg, respectively. Soil microbial biomass carbon and nitrogen were lower than CK after nitrogen application. With the increase of the amount of Sesbania cannabina overturning, soil pH decreased gradually, while soil total water-soluble salt increased with the increase of nitrogen application level or the amount of Sesbania cannabina overturning, but it was significantly lower than that before planting. After the overturning of Sesbania cannabina under different nitrogen application treatments, Proteobacteria, Acidobacteria, Bacteroidetes, Actinobacteria and Chloroflexi, which have certain organic degradation function were the dominant phyla in the soil. The relative abundance showed a certain change trend with the increase of the turnover amount of Sesbania cannabina, but the change of bacterial community structure was not obvious. There was a certain correlation between the dominant bacteria at phylum level with soil carbon, nitrogen, pH and total water-soluble salt. Among them, the correlation of Bacteroidetes, Firmicutes, Cyanobacteria, Chloroflexi and Mitrobacter with soil organic carbon and total nitrogen were significant. In summary, the returning of Sesbania cannabina to the field under SN2 can significantly increase the contents of soil organic carbon and total nitrogen in the tidal flat, the returning of Sesbania cannabina can significantly reduce soil water-soluble salt, and the main dominant soil bacterium groups are beneficial to green manure degradation and soil fertilization, thus can improve soil micro-ecological environment of coastal area.