The organic carbon isotope composition in loess deposits is closely related to paleoclimate, which is of great significance to the study of regional environmental evolution. The loess deposit of the Last Glacial Period in Shengshan Island in the East China Sea off the East China was studied. The magnetic susceptibility and element geochemistry were analyzed, based on which the organic carbon isotope composition was scrutinized. Results show that since the Last Glaciation, the organic carbon isotope composition in the loess fluctuated from −21.63‰ to −27.56‰ on average of −24.88‰. In general, the value of organic carbon isotope decreased with the increase in burial depth of the loess. In addition, the relative abundance of C3
vegetation in Shengshan Island was estimated by using the end-member method. It was revealed that the C3
plants dominated in the island since the Last Glacial Period, and C4
plants were very limited in the contribution to the organic carbon isotope in the loess deposit. In comparison with the oxygen isotope data of the cave stalagmites in Nanjing, marine sediments from Sulu Sea, and Antarctic Vostok ice core, the loess was deposited during the interstage of the Last Glaciation, and the fluctuation in organic carbon isotope value was resulted mainly from the responses of local ancient C3
plant-dominated vegetation to the variation of precipitation condition. The precipitation was the main influential factor on the organic carbon isotope variation during the period. In the last deglaciation stage, the paleo-temperature favored C4
plants booming, thus the relative abundance of C4
plants increased, and so did their contribution to the fluctuation of organic carbon isotopes in the loess deposits in the island.