The dietary composition and its ontogenetic shift of the Japanese grenadier anchovy () in the Qingcaosha Reservoir of Shanghai, China were examined using stable carbon and nitrogen isotopes to complement stomach content analysis. The cluster analysis showed that the gravimetric percentage (%) differed significantly between small-sized (body length < 99 mm, SS), medium-sized (between 99 mm and 150 mm, MS), and large-sized (>150 mm, LS) anchovy individuals. Occurrence rate (%) and percent index of relative importance (IRI%) did not differ between SS and MS individuals, which of them were significantly different from those of LS individuals. There was no significant difference in numerical percentage (%) between the three body length classes. Combining the results of all the four indices, zooplankton was the main dietary source for SS anchovy, and shrimp and fish were the main dietary sources for LS anchovies. For MS anchovy, zooplankton made up the largest proportion of the diet by %. A dual stable isotope mixing model showed that shrimp and fish made the largest contribution to the diet of the LS anchovy. The N value increased linearly with increasing body length, and C varied as a quadratic polynomial function of body length. The N[(15.18±0.79)‰] and trophic level (TL:3.63±0.23) of LS anchovy were significantly larger than those of the MS anchovy[δ¹³C did not differ significantly between the LS and MS classes (>0.05). Combining the stomach content and stable isotope analysis, an ontogenetic dietary shift was noted from the pelagic zooplankton to benthic organisms as primary carbon sources before and after the diet shift. This indicates the dominant species of anchovy could play an important role in the coupling of the pelagic and benthic food webs of the reservoir.