| Caohai Plateau Wetland is a National Nature Reserve. The characteristics of stable mercury isotopic compositions in the food web were studied by comprehensively analyzing the concentrations of Hg species (MeHg, THg), δ13 C, and δ15N, and the isotopic compositions of Hg in different aquatic organisms. The main results are as follows:all samples of the food web show mass-dependent fractionation (MDF) and mass-independent fractionation (MIF) and a negative δ202 Hg(-0.93‰±1.32‰, n=14)and positive △199 Hg(0.79‰±0.76‰, n=14). The δ199 Hg values are significantly positively correlated with δ15N (r=0.65, P<0.05) and the δ202 Hg values also exhibit a positive correlation with δ15N, except for Myriophyllum spicatum(δ15N=-1.88‰), indicating that the bioaccumulation of mercury leads to an enrichment with heavier isotopes. The △199 Hg values increase with δ15N(r=0.67, P<0.05). Nevertheless, the △199 Hg values correlate with% MeHg (r=0.58, P<0.05), indicating that the increase of the MIF level in the samples with the food web might be related to% MeHg.