Abstract: Soil carbon pool and abundlance of organic carbon stable isotopes were investigated at the Yangtze River Delta region across which the coastal herbaceous plants (Cynodon dactylon, Trifolium repens, Festuca arundinacea and Imperata cylindrica) occupy commonly, and this pool was quantified with Stable Isotope Mixing Model as the contribution of coastal grassland ecosystem. The results of this study show:  (1) the average contents of total carbon, organic carbon and dissolved organic carbon correlate significantly with each other and decrease with increasing soil depth from 2211 g·kg-1, 1144 g·kg-1, and 5395 mg·kg-1 to 1557 g·kg-1, 707 g·kg-1, and 1947 mg·kg-1, respectively.  (2) On the contrary, while the δ13C values of C3 plants increased with the depth increasing in the vertical direction, the δ13C values of C4 plants decreased with the depth. From upper layer to deeper layer, the average δ13C value of two types of plants change from -2524‰,-2233‰ to -2435‰,-2327‰; (3) The stable isotope mixture model found the contribution rate of different plants to soil organic carbon and the organic carbon accumulation rate was completely different among those four plants. The contribution rate of Cynodon dactylon, Trifolium repens, Festuca arundinacea, and Imperata cylindrical is  1219%, 1434%, 3595% and 1851% with the cumulative rate of 6279 g·m-2·a-1, 7534 g·m-2·a-1, 18184 g·m-2·a-1, and 9770 g·m-2·a-1, respectively