The purpleback flying squid (Sthenoteuthis oualaniensis), which is one of the main fishing objects for light attracting falling-net fishing vessels, has abundant resources in the South China Sea. In this study, samples of medium-form and dwarf-form of S. oualaniensis were collected in the Nansha Islands area. Traditional stomach contents analysis and carbon and nitrogen stable isotope analysis were used to study feeding habit, trophic level, trophic niche and their relationship with feeding organisms. Consequently, S. oualaniensis mainly fed on fish, cephalopods and crustaceans, and the food composition was different at different growth stages. The medium-form smaller than 90 mm ML and the dwarf-form smaller than 100 mm ML fed on zooplankton and small fish. The medium-form ranging 90-129 mm ML and the dwarf-form ranging 100-119 mm ML fed mainly on crustaceans, cephalopods and fish. The food intake was mostly by fish and cephalopods for the medium-form larger than 130 mm ML and the dwarf-form larger than 120 mm ML. Besides, the δ¹³C value of the medium-form ranged from –19.61‰ to –18.10‰ and the δ¹⁵N value ranged from 7.17‰ to 10.13‰. The δ¹³C value of the dwarf-form ranged from –19.63‰ to –17.81‰ and the δ¹⁵N value ranged from 6.48‰ to 10.12‰. Analysis showed that there was an overlap of trophic niches between the medium-form and the dwarf-form, which indicated that the squids had competition for food resources. The trophic niche of dwarf-form was larger than medium-form, and there was lower overlap between the dwarf- male group and the other three groups. In addition, the mean trophic level was 2.97 and ranged from 2.54 to 3.41 for the medium-form, and the mean trophic level was 2.87 and ranged from 2.34 to 3.41 for the dwarf-form. The trophic level of the dwarf-male group was relatively low and quite different compared with other groups, which may be closely related to body size.