A field experiment was conducted to explore the effects of overturning Sesbania cannabina on the improvement of soil in coastal area, the biomass, carbon and nitrogen contents of Sesbania cannabina under different fertilization treatments (CK, N 0 kg/hm²; SN1, N 90 kg/hm²; SN2, N 135 kg/hm²; SN3, N 180 kg/hm²; SN4, N 225 kg/hm²; SN4+OF, N 225 kg/hm²+organic fertilizer), microbiological biomass, carbon and nitrogen contents of soil were determined, and the correlations between soil properties and plant indexes were analyzed. The results showed that SN3 got the highest total biomass, and nitrogen and carbon contents in twice mowing of Sesbania cannabina. SN4 + OF had the highest contents of soil organic carbon, total nitrogen and fixed ammonium, which were 6.44 g/kg, 0.62 g/kg and 40.1 mg/kg, respectively. SN2 had the highest contents of active organic carbon, nitrate nitrogen, ammonium nitrogen, microbial biomass carbon (MBC) and nitrogen (MBN) and CPMI. SN4 + OF and SN2 got the highest ratios of soil C/N and MBC/MBN. In conclusion, SN3 can significantly increase the biomass, carbon and nitrogen contents of Sesbania cannabina, and SN2 and SN4+OF have the better effect on soil improvement.