The exotic C₄ grass Spartina alterniflora was introduced in China in 1979 and transplanted to Xinyanggang coastal wetlands in North Jiangsu in 1989. Then it had expanded rapidly on the mudflats and become one of the dominant vegetation of local salt marsh. S. alterniflora invasion has affected comprehensively on biodiversity, sediment characteristic, landscape and especially on soil carbon cycling of the local ecosystem. The effects of S. alterniflora invasion on soil carbon cycling in China had been widely demonstrated; however, little is related to how the S. alterniflora seaward invasion influences the fractions and sources of soil organic carbon (SOC). The purpose of this study was to: (1) compare SOC, recalcitrant carbon (RC) and soil labile carbon (LC) pools at 0-20 cm depth between S. alterniflora salt marshes of different seaward invasion period, and (2) evaluate the effect of S. alterniflora seaward invasion on SOC, RC and LC sources through stable carbon isotopic analysis ofδ¹³C. In June of 2011, five sampling sites of mudflat (MF), S. alterniflora salt marshes of 1-year (SAF-1), 3-year (SAF-3), 5-year (SAF-5) and 12-year (SAF-12) along Xinyanggang coastal wetland were arranged and three 1 m×1 m sampling plots were randomly selected at each site. The different invasion time of S. alterniflora was identified based on historical marks. Three topsoil (0-20 cm) cores were taken at each sampling plot and the soils are mixed for the determination. The fresh plant and litters of S. alterniflora were collected in each plot. The soil samples were analyzed for SOC, RC, LC, and total nitrogen(TN) concentrations, C/N ratio, and δ¹³C of SOC and RC. RC and LC were determined following the acid hydrolysis procedure. The leaves, roots and litters of plant were analyzed for δ¹³C. Meanwhile, soil water content, soil salinity, grain size and plant biomass were determined. The results showed that: (1) The SOC, RC and LC were between 0.82-7.60 mg/g, 0.58 -4.02 mg/g and 0.23-3.58 mg/g, respectively, and increased as the same order of SAF-5 > SAF-12 > SAF-3 > MF > SAF-1. The highest value of SOC occurred in SAF-12, where the SOC accumulated by1.8 tC / hm² per year. (2) The SOC, RC and LC derived from S. alterniflora were between 0.06-3.01 mg/g, 0.04 -1.06 mg/g, and 0.03-2.00 mg/g, accounting as 5.75-47.40%, 6.77-31.77% and 3.20-64.40% of total SOC, RC and LC in soil, respectively, and increased with the same order as SAF-12 > SAF-5 > SAF-3 > SAF-1 > MF. (3) The contribution of S. alterniflora to SOC, RC and LC significantly increased with invasion time (P<0.01). S. alterniflora invasion increased soil LC more than RC. The fractions of organic carbon in soils performed a significant positive correlatation to S. alterniflora biomass and sediments grain size. (4) The fractions of SOC were changed significantly by S. alterniflora derived SOC inputs (P<0.01). The results indicated that exotic S. alterniflora was of great importance to carbon sequestration in the Yancheng coastal wetland because of the high input of the organic carbon from S. alterniflora and alteration of sedimentary environment by S. alterniflora.