Distribution of particulate organic carbon (POC) with depth was studied to elucidate the mechanism for vertical distribution of salt marsh POC, based on analyses of POC content, stable carbon isotope and grain size for samples of three cores from high marsh, middle marsh and bare flat of Chongmingdongtan Salt Marsh in the Yangtze River Estuary. Results indicated that soil particulate organic carbon existed mainly in the fine grains with sizes less than 0.016 mm, and POC content was susceptible to changes of contents of grains in size between 0.002 mm and 0.004 mm. It was inferred that salt marsh POC originated mainly from suspended particulate matter of the Yangtze River, based on results of stable carbon isotope analysis and the correlations between POC content and contents of fine grains in different size fractions (<0.016 mm) for soil samples from different cores. The correlations between soil POC contents and contents of grains in different size fractions indicated that grain size characteristics of samples from high marsh were similar to those of samples from middle marsh. Samples from bare flat were similar in part to those from high marsh and middle marsh in the correlations between soil POC content and contents of grains in different size fractions, with main differences shown by grains in size fractions greater than 0.016 mm. This might be due to the dynamic depositional processes at locations with different altitudes in the salt marsh. Vegetation provided considerable contributions to soil POC of core samples from high marsh and middle marsh, with the greatest value of 55.6% of soil POC contents in some depth sections, which was controlled predominantly by depositional processes on the tidal flat. The characteristic structure of alternate muddy laminae and sandy laminae originated from dynamic depositional processes on tidal flats, and exerted great influences on the distributions of soil POC with depth, due to the excellent protecting effects of the strata structure on soil POC. The dynamic depositional processes on tidal flats were therefore the key factor constraining the distributions of soil POC with depth.