Abstract: This study was carried out at a seabeach salt marsh in Donghai Farm, Nanhui county, Shanghai. Population density, height growth and biomass of Scipus mariquetep, as well as the interrelationships between them and with the environmental conditions, were described. The research results showed that at the relatively stable habitats(zone A and B), the population density, height growth and biomass dynamics during the growing season was in accodance with logistic growth. Three phases could be recognized for the population density and biomass dynamics, and the-3/2 power law of natural thinning was applicable to the populations in phase Ⅱ. Zone B was the optimum area for population growth of Scirpus mariqueter. The habitat conditions at the zone C were quite unstable and the population density and biomass dynamics was fluctuated drastically. Under different environmental conditions, the relative roles of density-independent and density-dependent factors on population dynamics differed greatly. At the relative stable habitats(zones A and B), the density-dependent factors were the major factors to determine population dynamics, while at the unstable habitat (zone C), the density-independent factors were the major factors.