In response to the characteristics of clay saline-alkali soil in the Xuwei New District of Lianyungang, Jiangsu Province, this study applied soil conditioner and adopted in-situ planting of oleander (Nerium indicum), with bare land without vegetation as the control, to investigate the improvement effects and mechanisms of oleander planting on clay saline-alkali soil at different depths (0-20 cm, 20-40 cm and 40-60 cm). The results showed that planting oleander not only significantly reduced soil salinity and Cl^- content in the 0-40 cm soil layer but also significantly increased the content of nutrients such as soil organic matter, total nitrogen, and available phosphorus in the 0-40 cm soil layer. Moreover, the combined application of soil conditioner and oleander planting had a synergistic effect on the improvement of soil physiochemical properties. Planting oleander significantly altered the composition and structure of soil microbial communities across different soil layers, although soil microbial species evenness, richness and diversity did not increase significantly. At the phylum level, oleander planting significantly reduced the relative abundance of Crenarchaeota but increased the relative abundance of Actinobacteria. At the genus level, the abundances of Flavisolibacter, Marinobacter, Ohtaekwangia, Pseudomonas and Salinimicrobium were significantly increased by oleander planting. Overall, oleander can effectively improve coastal clay saline-alkali soil in Jiangsu by reducing salt and alkali levels, enhancing soil fertility, and promoting microbial community differentiation, and the combined application of soil conditioner and oleander planting exhibits a synergistic effect on improving the physicochemical properties of saline-alkali soil.