Abstract: The burial rate of organic carbon in a bay is closely related to its geological carbon sink capacity. It is an important challenge to evaluate the ocean carbon sink capacity to support the “dual carbon” goal (carbon peaking and carbon neutrality). Taking the surface sediments and sediment core samples collected in 2019 in the Sanmen Bay, Zhejiang Province as the research object, the typical characteristics of the complex seabed topography, strong sea-land interaction, and active human activities in the bay area were studied comprehensively using particle size analysis, organic geochemical analysis, and ²¹⁰Pb and ¹³⁷Cs dating methods. A carbon sink grid calculation method based on the division of bathymetric topographic in the study area was explored, and the bay was divided into shallow tidal flat, deep-water channels, and underwater plain. In addition, the burial rates of organic carbon in different sedimentary environments were obtained. Therefore, a geological carbon sink evaluation model of the Sanmen Bay was established by using inverse distance weighted spatial interpolation. The calculation results show that the burial rate of organic carbon in the study area was 64.04 gC/m²·a. The annual buried amount of organic carbon was 89.71 GgC. Among them, the burial rate of organic carbon in the shallow tidal flat and deep-water channel areas was relatively high at 74.02 and 80.48 gC/m²·a, respectively, and that in the underwater plain area was 52.93 gC/m²·a.