To investigate the growth dynamics and carbon sequestration effects of kelp under different cultivation densities, this study used the traditional cultivation density of 100 ropes per raft (K0) in Sanggou Bay as the control group. During the tender stage of kelp (January), the cultivation density was reduced to 67 ropes per raft (K1) and 50 ropes per raft (K2), respectively. Methods such as on-site instrument monitoring, in-situ perforation sampling, UV-Vis absorption spectroscopy, and organic carbon analysis were employed to measure the light attenuation rate, absorption spectra of colored dissolved organic matter (CDOM) in surface seawater, elongation rate, and thickening rate of kelp blades in different density areas of Sanggou Bay. Differences in kelp growth dynamics under varying densities were analyzed, and carbon sequestration under different cultivation densities were estimated. The results showed that: (1) The light attenuation rates in the 0–1 m depth layer differed significantly among the three cultivation density areas. (2) The highest blade elongation rate in all three density groups occurred during the E3 stage (January–March), while the highest thickening rate appeared during the E4 stage (March–April). By the end of the experiment (June), the wet weight per individual kelp in the K1 and K2 groups was significantly greater than that in K0, and the total wet weight of kelp cultivation in the K1 group was significantly higher than that in the other two groups. (3) The absorption coefficient a(355) of colored dissolved organic matter (CDOM) at 355 nm in the K0 and K1 cultivation areas was significantly higher than that in K2 from March to June, while the SUVA254 values at the K0 and K1 stations in March and June were significantly higher than those at K2. By the end of the experiment, the total carbon pool contribution of the K1 group increased by approximately 15.19% compared with K0. The findings indicate that reducing kelp cultivation density to a reasonable level can significantly enhance kelp growth, yield, and the carbon sequestration of the cultivation system. This provides data support and technical references for developing models to enhance carbon sinks through large macroalgae cultivation in shallow seas.