Wetland plants play an important role in the biogeochemical cycling of nutrients. Studying the absorption, distribution, and accumulation of nitrogen (N) and phosphorus (P) in plants is of great significance in understanding the key processes of nutrient cycling and their ecological functions. Therefore, field observations and laboratory measurements were used to study the dynamic changes in biomass as well as the N and P contents and their storages in Carex cinerascens in the spring growing season in Poyang Lake wetland. The results showed that there was a time-dependent increase in the biomasses of different components of C. cinerascens during the spring growing season whereas the aboveground biomass was higher than that of the belowground in each growing period. Furthermore, the accumulation rate of the belowground biomass was relatively constant whereas that of the aboveground and total biomasses was higher in the early growing stage than it was in the late growing stage. The N and P contents in the different parts of C. cinerascens decreased from March to April and then increased from April to May. The N content of the aboveground part was higher than that of the belowground part in each growing period, and the difference was significant in April and May (P < 0.01). The P content had similar distribution characteristics to those of the N content, but the difference was more significant in March and April (P < 0.01) than it was in other months. The N and P storages in C. cinerascens showed a significantly positive correlation with the biomass (P < 0.01), and their dynamics were determined based on the biomass variation of C. cinerascens. Further, the aboveground part of C. cinerascens contained the main N and P pools. The N/P ratio is an important ecological indicator that can be used to detect the nutrient limitation. Based on the theory of nutrient limitation, N may be a key nutrient factor that limits the primary productivity of C. cinerascens.