The coastal zone serves as a critical interface for the fixation and storage of organic carbon at the land-sea boundary, exhibiting significant spatiotemporal characteristics and patterns in organic carbon formation over millennia of cultivated soil development. This study aimed to analyze the vertical distribution characteristics and temporal evolution patterns of soil organic carbon (SOC) of sandy-silty coastal zones of the South Yellow Sea over millennia. Twenty one-meter-deep soil profiles were collected based on the position of the coastlines from Sheyang in the north and Dongtai in the middle of the study area. The spatiotemporal distribution characteristics of SOC mass fraction (SOCC), density (SOCD), and stock (SOCS) were analyzed, along with their correlation with soil physicochemical properties. Results indicated: 1) Over millennia, SOCC in coastal cultivated soil profiles ranged from 1.00 to 24.44 g·kg-1, while SOCD varied within 0.13-2.78 g·m-2, with both SOCC and SOCD decreasing with depth in the two regions. 2) The temporal accumulation functions of SOCS differed between the two regions, with SOCS in Sheyang showing a linear relationship with soil age, while in Dongtai, SOCS exhibited a logarithmic function, primarily due to differences in soil texture. 3) Significant correlations were found between coastal cultivated SOC and soil physicochemical properties, with Sheyang showing higher correlations between SOC and bulk density, total salinity, whereas Dongtai exhibited higher correlations with soil texture. This study provides important insights into the dynamic characteristics and long-term storage mechanisms of coastal cultivated soil carbon pools, offering valuable references for understanding these systems.