Abstract: In this paper, the biomass of different plants in winter and spring are estimated. The contents and variations of N and P of plants, surface layer and core sediments in different ecological zones are also compared and analysized. Subsequently, the dynamic variations of N and P, which influenced by absorption, accumulation and transformation of different plants, are discussed. The results indicate that: for the zonation of tidal flat and different physiology construction of plants, the N and P show different distribution patterns and seasonal variations in different ecological zones; plants, especially perennial Spartina anglica and Phragmites communis with high ratio of aboveground and belowground biomass, play more important role in contents and seasonal variations of N and P in the whole tidal flat sediments; the main mechanism that the plant influence the sediment content of N and P include the active transformation of N and P in plant body, the absorption effect of plant, and the release of N and P from the decomposed and mineralized deceased belowground rhizome that accumulated in the belowground sediments; the quantities of N and P, returning to the ground with withering in different ecologic zones, reach 149.4 t and 18.2 t from January to April, and the biogeochemistry cycle and ecological environment of the tidal flat can be seriously affected.