The aim of this study was to explore the feeding habits of Cultrichthys erythropterus by analyzing the changes and regularity of δ¹³C and δ¹⁵N in Gonghu Bay, Lishan Bay, and east Lake Taihu, using the stable isotope technology. The results showed that the values of δ¹³C and δ¹⁵N were positively correlated with the body length, and the δ¹³C and δ¹⁵N values. The trophic level in large C. erythropterus(>138 mm) individuals was significantly higher than that in the smaller ones (<138 mm), which indicated that C. erythropterus underwent size-related diet shift switching from small zooplankton to fish due to predation ability strengthening and mouth crack increasing. Obvious spatial heterogeneity could be observed in the δ¹³C and δ¹⁵N values of C. erythropterus in the three areas. C. erythropterus in east Lake Taihu(-25.17‰±2.56‰), affected by aquaculture, presented lower values of δ¹³C than those in Gonghu Bay(-23.11‰±1.05‰) and Lishan Bay(-22.73‰±1.31‰), and were generally smaller in body sizes, revealing that there were certain correlations between the population distributions and geographic locations. The C. erythropterus was fed on diets according to different habits, and preferred to large animal bait under the environment of high concentrations of blue-green algae in order to reduce the feeding pressure of zooplankton, and indirectly controlled the development of eutrophication. Furthermore, we also studied the influence of C. erythropterus on ecosystem. The results of this study can provide the theoretic basis for the protection and rational allocation of resources of the region.