Clarifying the stability of salt marsh soil organic carbon sinks is of great significance when scientifically evaluating the carbon sink potential of “blue carbon” in coastal zones. The chemical structure and composition of Soil Organic Matter (SOM) are closely related to the stability of soil organic carbon sinks. This study focused on four reclamation areas with different construction times in the eastern part of Chongming Island, China. Solid-state ¹³C Nuclear Magnetic Resonance (¹³C NMR) was used to investigate the characteristics of SOM chemical structures in buried salt marsh samples obtained by drilling and topsoil samples acquired from woods around the drilling cores (CM2, CM4, CM5, and CM6). The purpose of this study was to elucidate the variations in the chemical structure of SOM during its turnover. The results showed that \mathrm{①} the variations in soil organic carbon functional groups from the surface to the deep layers exhibited similar trends for the four reclamation areas, despite different construction times. The alkyl carbon and aromatic carbon proportions showed an increasing trend whereas those of alkoxy carbon and carbonyl carbon showed a decreasing trend from the topsoil samples to the buried soil samples. The alkyl carbon/alkoxy carbon, aromaticity, and hydrophobic carbon/hydrophilic carbon of the buried salt marsh soil samples were greater than those of the topsoil samples in the woods around the drilling cores. The stability of the SOM chemical structure increased with burial depth. \mathrm{②} From west to east, alkyl carbon/alkoxy carbon, aromaticity, and hydrophobic carbon/hydrophilic carbon were all less than 1 and showed a gradually increasing trend, but aliphatic carbon/aromatic carbon gradually decreased in the topsoil samples in the woods around the drilling cores. The overall decomposition degree of organic matter in topsoil samples from the woods was low and showed a gradually increasing trend from west to east, which meant that SOM stability gradually improved. This was because the planting time for trees gradually became earlier and the development time for topsoil became longer in the woods from west to east. \mathrm{③} From east to west, the proportions and ratios of organic carbon functional groups in buried salt marsh samples showed that SOM stability increased with reclamation time. In summary, along with soil development, the decomposition degree of salt marsh SOM continued to improve, with an increasing proportion of refractory SOM components and a decreasing proportion of labile SOM components. In addition, SOM stability continued to increase. The changes in the chemical structure and composition of SOM in the reclamation areas, constructed in different years along the Yangtze River estuary, were studied using solid-state ¹³C NMR and the results improve understanding about the temporal trend and chemical mechanism underlying soil carbon sink stability.