Coastal wetlands are known for their significant capacity as carbon sinks, with different types of vegetation playing a crucial role in both sourcing and storing organic carbon in sediments. In this study, sediment core samples (1-meter length, sampled at 10 cm intervals) were collected from four different tidal flats on Maoyan Island, including bare mudflat, mature mangrove, young mangrove, and Spartina alterniflora wetland. The particle size, total organic carbon (TOC), total nitrogen (TN) and other parameters of each layer of sediment were measured at 10 cm interval. The source, storage and influencing factors of sediment organic carbon were analyzed and discussed. The results showed that: (1) The average TOC content in sediments from bare mudflat, mature mangrove, young mangrove, and Spartina alterniflora wetland were 0.71%±0.03%, 0.76%±0.16%, 0.69%±0.12%, and 0.83%±0.09%, respectively. Vegetated flats had significantly higher TOC content than that in bare mudflat in the 0-20 cm layer, while Spartina alterniflora wetland had higher TOC content than those in other tidal flat types in the 20-100 cm layer. (2) Among the 1 m of sediment from Maoyan Island tidal flats, the carbon storage in the Spartina alterniflora wetland was the highest, reaching 5.79 kg/m2, followed by the mature mangrove forest (5.61 kg/m2), the young mangrove forest (4.95 kg/m2), and the bare mudflat (4.84 kg/m2) with relatively lower organic carbon storage. The coverage of Spartina alterniflora and mangroves enhanced the carbon storage capacity of tidal flat to a certain extent. (3) The organic carbon in the tidal flat sediments of Spartina alterniflora was mainly from terrigenous sources, accounting for 57.75%; the contribution of native plants accounted for the largest proportion in the mature mangrove sediments, accounting for 32.65%; the organic carbon in the sediments of young mangroves and bare mudflat was dominated by marine sources, accounting for 61.47% and 50.45%, respectively.