Salpa thompsoni is an important tunica filter feeder in the Southern Ocean, which plays a significant role in the Antarctic marine food web. S. thompsoni and Antarctic krill (Euphausia superba) compete for similar diets. Therefore, in order to study the factor influencing food composition of S. thompsoni in the fishing ground of Antarctic krill fishery, based on the specimens collected from the Bransfield Strait from March to April 2020, this study identified the food composition of this species, and further analyzed the difference in feeding habits between months and life history stages. The results showed that S. thompsoni mainly fed on diatoms and zooplankton, and a small number of ciliates and protozoa. C. pennatum is dominant in the diet of S. thompsoni. Seasonal differences occurred in diet of S. thompsoni. Specifically, the feeding quantity of diatom, i.e., C. pennatum, which is dominant in March, decreased significantly at the end of April. The diet composition of S. thompsoni changed significantly between solitary and aggregating stages. This study is expected to provide a reference for the study of feeding habits of S. thompsoni in the Southern Ocean, and to provide basic information for clarifying the trophic relationship between S. thompsoni and Euphausia superba.