Under coastal reclamation, the physicochemical environment of wetland soil in the coastal zone experiences significant alterations, impacting the soil organic carbon (SOC) content and the carbon sequestration capacity of the wetland ecosystem. In order to reveal the influence of coastal reclamation and its history on the carbon sink function of coastal wetlands, this study focuses on the eastern coastal zone of Chongming Island in Shanghai as a representative research area. This study compares changes in soil physicochemical factors, SOC contents, Dissolved Organic Carbon (DOC), Microbial Biomass Carbon (MBC), and Easily Oxidized Organic Carbon (EOC) are analyzed to understand how coastal perimeter and its history influence SOC and active carbon components in coastal wetlands. Based on these findings, suggestions for protecting and managing coastal wetlands through coordinated land-sea approaches are proposed. The results showed that: (1) After coastal reclamation, the soil Electrical Conductivity (EC), Total Nitrogen (TN) content, clay decreased, with EC progressively lowering as reclamation duration increased, while TN content and clay first decreased and then increased. (2) After coastal reclamation, the contents of SOC and DOC initially declined and subsequently exhibited an increasing trend as the years since reclamation progressed, while MBC and EOC initially surged post-reclamation, then decreased, and eventually increased with the passage of fencing years, correlating positively and significantly among SOC, DOC, MBC, and EOC.(3) The ratios MBC/SOC, DOC/SOC, and EOC/SOC in the coastal zone were significantly higher than those in the inland wetland, showing an initial increase and subsequent decrease with reclamation years, suggesting that SOC stability decreased initially after reclamation and then improved over time. The findings of this study are valuable for comprehending the impact of enclosure on soil organic carbon in coastal ecosystems, and provide a scientific foundation for enhancing the carbon sink function and implementing effective management strategies for coastal wetland protection under integrated land-sea planning. This aligns with China's objective of achieving "double carbon" goals.