To reveal the relationship between lignin degradation and release of Carbon (C), Nitrogen (N), Phosphorus (P) and their response to the change of ground water level, a field experimental site with 200 m×300 m was setup in the beach of Baisha Lake, a typical shallow lake of Poyang Lake Wetlands. The site included 4 ground water levels: GT-A, GT-B, GT-C, and GT-D (moving from shore to center). Litter bag technique was applied to simulate the Carex cinerascens decomposition under different groundwater level gradients. The results showed that the rise of groundwater level promoted the lignin degradation and release of C, N, and P (CRRI, NRRI, PRRI) after 15 days' decomposition. The CRRI, NRRI, and PRRI had significant positive correlations with lignin decay rates during 60-90 days, and the degree of these correlations increased for the first 60 days and then declined. Lignin degradation and the release of C, N, and P were influenced positively and consistently by soil pH, moisture, soil microbial biomass carbon, and soil microbial biomass nitrogen. This study provides useful insight for plant decomposition and biogeochemical processes of wetland ecosystem, and provides basis for the scientific management of the wetland nature reserves.