The relationship between abundance and trophic level is an ecosystem-level indicator of anthropogenic influences and environmental changes, and can reveal the differences in ecosystem structure and function. The present study investigated the relationship between the biomass and δ¹⁵N value of size-fractioned particulate organic matters, and compared the structure of pelagic food web of Daya Bay using the method of trophic spectrum. Sample sites were selected: S1 in the mouth of Daya Bay, S3 in the aquaculture area, S5 in the effluent area of Nuclear Power Plant, S8 in the inner area of Daya Bay, S9 in the pelagic area, and S11 in the mouth of Fanhe Cove. Plankton samples were separated into four size fractions: 1.2-100 μm, 100-212 μm, 212-500 μm, and > 500 μm. The average biomass of four size fractions were 12.04 mg dry wt/L, 2.92 mg dry wt/L, 0.032 mg dry wt/L, and 0.015 mg dry wt/L in the six sample sites respectively. The average δ¹⁵N values of four size fractions were 5.58‰, 5.08‰, 7.36‰, and 8.16‰ respectively. The results showed that the δ¹⁵N values of size-fractioned plankton generally increased with the size class except the 100-212 μm fraction. The average δ¹⁵N value of 100-212 μm fraction was lower than the 1.2-100 μm fraction, which may be attributed to the more micro-zooplankton and detritus in the 1.2-100 μm fraction. The δ¹⁵N of each size fractions was low in the mouth of Daya Bay (S1) and pelagic area (S9). The tendency of δ¹⁵N was S1 < S9 < S11 < S3 < S5 < S8 in the fractions of 1.2-100 μm and > 500 μm. In the fraction of 100-212 μm, the tendency of δ¹⁵N changed to S5 < S1 < S9 < S11 < S3 < S8. In the inner zone of Daya Bay (S8), δ¹⁵N values were higher in each fractions than those of other sites. Previous studies suggested that anthropogenic nutrient inputs can increase the δ¹⁵N value of particulate organic matters and their δ¹⁵N value can well indicate the anthropogenic euthrophication in aquatic ecosystems. Our results showed that the δ¹⁵N value of 1.2-100 μm fraction was most suitable to indicate the variation of environmental condition among the size fractions, which was significantly correlated with Chl a, SiO₃, NO₃, DIN and DIN/DIP. We constructed the trophic spectrum according to the biomass and δ¹⁵N value of size-fractioned plankton in the six sites. The average slope of trophic spectrum was -0.95. The slope of trophic spectrum was steepest in S1 with a value of -1.3 and was gentlest in S5 with a value of -0.7. The slope value of trophic spectrum were S1 < S3 < S9 < S11 < S8 < S5. There were significantly negative correlation between the slops of trophic spectrum and DIN and DIN/DIP. The concentration of DIN and Chl a were highest in the steepest S1. It suggested that the high nutrient concentration and primary production favor to a steeper trophic spectrum and a lower predator/prey ratio in the pelagic food web in winter of Daya Bay.