A great research effort addressed the effect of plant litter decomposition on nutrient cycling in terrestrial and aquatic ecosystems. Environmental factors such as temperature and moisture content related to the decomposition process have been widely studied, and among the hydrological condition is more important and has quite complicated influence. In the present study, we investigated the effects of water conditions on decomposition and nutrient (N and P) dynamics of Carex lasiocapa litters using litter bag method in the Sanjiang Plain of Northeast China. The litter decomposition study was carried out in a typical dish-shaped pit along a water table gradient from May in 2009 to May in 2010. 60 nylon bags (approx 1mm diameter) with 8g C. lasiocapa litters were placed near the soil surface or the grass layer in the marsh under unflooded or flooded conditions, with 12 under every water condition. 3 bags in each site were taken back to the laboratory every time and then the mass losses and nutrient elements (N and P) contents were analyzed. By the end of decomposition, mass loss of C. lasiocapa litters was 34.99% and 27.28% under non-waterlogging and seasonally inundated conditions, respectively. In the permanently inundated conditions, with increased water table, litter mass loss ranged from 26.99% to 30.67%. The results showed that during the growing season (from May to September), C. lasiocapa litter decayed quickly in the flooded conditions. With increased water table gradient, the mass losses were 16.09%、24.25%、23.53% and 26.60% in non-waterlogging, seasonally inundated and permanently inundated condition, respectively. However, during the non-growing season, though higher water table accelerated the C. lasiocapa litter decomposition slightly in the inundated environment, flooded conditions significantly inhibited the litter decay compared to the dry condition (P<0.05). With increased water table gradient, the litter mass losses were 18.90%, 3.02%, 3.46% and 4.03% in non-waterlogging, seasonally inundated and permanently inundated conditions, respectively. In the growing season, litter N concentration and total N content increased gradually in the early 122 days of decomposition. Waterlogging conditions enhanced litter N immobilization, and litter N concentration under the deepest standing water condition was significantly higher than that under the dry condition (P<0.05). However, litter N concentration and total N content declined in the freeze-thawing season, and all the litters under flooded conditions released N after 122 days of decomposition. In addition, seasonal inundated conditions significantly inhibited litter N accumulation, and litter N content was significantly lower than the initial value after 360 days of decomposition (P=0.01). Moreover, litter P content significantly increased after 61 days of decomposition, especially in the flooding conditions. However, litter P enrichment had no correlation to water table depths. For all litter decomposition treatments, litter P concentration and total P content decreased after one year of decomposition, but did not vary with water table (P>0.05). Our results suggested that, in the Sanjiang Plain of Northeast China, the effect of water table on litter decomposition varied with decomposition stage, and flooding conditions stimulated litter decomposition in the growing season in the freshwater marshes.