Through the sample analysis of soils in different wetland vegetation community types in Mengjin part of Yellow River wetland, this article discussed the effects on the soil sedimentary characteristics of different wetland vegetation communities in lower riparian after the construction of Xiaolangdi Reservoir, and studied the spatial variation characteristics of the organic matter in riparian wetland soil. The results showed that: influenced by the construction of upriver Xiaolangdi Reservoir, there is great variation in the soil sediment layer amount and thickness of different community types in riparian wetland. The feature of the vegetation succession from hygrocolous to terrestrial is distinct. The soil sediment velocity increases and the riparian wetland continuously encroaches toward the river. Organic contents vary greatly among different vegetation communities and soil layers; The organic matter content of the upper layer soil in the typical wetland communities is obviously higher than that of the deep sediment soil; The fact that the soil organic matter in riparian wetland varies obviously and is controlled by external conditions fully indicates the fragility of riparian wetland.