Water level, an important hydrological factor in shallow lakes, has great influence on chemical processes. Wetland plants have become an important means of ecological restoration, but the problem of producing pollution after the plant decaying has been paid extensive attention. Base on the observation of water nutrients concentration of Lake Baisha, the monthly changes of total organic carbon (TOC), total nitrogen (TN), ammonium nitrogen (NH₄⁺-N) and nitrate nitrogen (NO₃^--N) concentrations were analyzed. Stable isotope analysis was applied to track the major source for particulate organic matter (POM) in the water. Combined with the decomposition simulation experiment performed in Lake Baisha, we revealed the specific time of the decomposition stage of Carex cinerascens in natural state, and studied the interaction effect of water level and decomposition process on water nutrients. Results showed that TOC, TN, NH₄⁺-N and NO₃^--N concentrations in lake water from Jan. to Apr. during the dry season were significantly higher than those during other periods. Carex cinerascens had the highest decay rate during the early 60 days. Under natural state, decomposition of Carex cinerascens begin at December according to the exponential decay model. δ¹³C value of water POM was similar to Carex cinerascens, while had significant differences with Vallisneria natans and alga. Litter decomposition had a certain effect on TN, NH₄⁺-N and TOC concentrations in water. Water level had obvious effect on NO₃^--N concentration in water. While water level and litter decomposition had accumulation effects on TN and NH₄⁺-N concentrations.