Mantle CO2 is widely distributed throughout the basins of the world. Risks are associated with its accumulation that have recently attracted attention in the oil and gas exploration industry. It is, therefore, necessary to determine the mantle CO2 distribution in a target region, which involves determining the locations of the CO2 sources, migration pathways, and areas of accumulation. In this paper, the potential accumulation of mantle CO2 across the East China Sea (ECS) basins is analyzed using the source and migration pathways. The results show that areas of Moho uplift (including paleouplift) correspond to areas of CO2 sources (including paleosources). Steep extensional faults, which penetrate the lower crust and upper mantle, and diapiric structures likely provide for mantle CO2 migration. These results can be used to develop general principles for locating mantle CO2 sources and pathways in rifted basins. Within the ECS basins, the Lishui, Jiaojiang, and Diaobei sags are considered high-risk areas for CO2 accumulation. The Xihu and Fuzhou sags are medium-risk areas, and the remaining sags are low-risk areas. The areas along the Lingfeng, Yandang, Fuzhou, and Xihu-Jilong faults are potential CO2 accumulation areas. This study indicates that the accumulation of mantle CO2 in the ECS basins can be predicted from the Moho depth distribution, fault systems, and traps across the region.