Purple sea urchin is an important benthic species in the western Pacific Ocean. In recent years, environmental disruption and over-exploitation have damaged the resources of this species. Stable isotope analysis has become a powerful tool for studying the feeding habits of marine animals, and this method can overcome some of the limitations associated with traditional stomach contents analysis. Carbon (δ¹³C) and nitrogen (δ¹⁵N) were used as tools to evaluate the stable isotopic characteristics, trophic levels, shell diameter, and feeding habits of , from Daya Bay on the south coast of Guangdong Province, using data collected in August 2015. In the present study, the trophic levels of 15N. Average δ¹³C of N was (9.14±0.38)‰, and the trophic levels ranged from 2.09 to 2.13 (mean 2.11); no significant correlation was found between the different shell diameters of the 15N (C values of the potential food sources were between-(9.93±1.59)‰ and-(20.76±1.42)‰, and their average δ¹⁵N values were between (-0.16±1.34)‰ and 14.99‰; the trophic levels ranged from 1.34 to 3.77. Cluster analysis showed that the food web among major species of Daya Bay could be classified into four trophic groups: suspended organic matter, primary producers and primary consumers, secondary consumers, and top predators. There were two main channels in the overall food chain: one was the grazing food chain, and the other was the detritus food chain. have a tendency to feed on detritus in August, when corals are scarce, macroalgae density is low, and the terrigenous materials present as particulate organic matter (POM) into Daya Bay follow a period of heavy rains. The main food resources of were: POM, sediment organic matter (SOM), the brown macroalgae , bacillariophyceae (diatoms), zooplankton and phytoplankton Among these, POM was the most important food item for , accounting for 67.3% of its food consumption; the other food categories accounted for 9.7%, 9.3%, 6.7%, 3.7% and 3.3% of its food composition, respectively. Considerable competition exists between and the benthos, which suggests urgent efforts are needed to improve this ecological environment as well as to increase the seaweed biomass. Variations in the trophic levels were related to differences in the isotope baseline, and temporal and spatial variations. There was no significant correlation between shell diameter of and its feeding habits, perhaps because its food source is relatively singular. Thus, it is evident that feeding habits play an important role in population resource recovery and ecological niches of the system.