Asian moon scallops (Amusium pleuronectes) were sampled by trawl vessels in the Beibu Gulf, South China Sea, from October 2014 to November 2015. Feeding habits and stable isotopic patterns of A. pleuronectes were examined using stomach contents and adductor muscles based on a conventional stomach analysis method and stable carbon and nitrogen isotope technology. The applicability of A. pleuronectes as a baseline indicator in marine ecosystems in the Beibu Gulf is discussed. The stomach content analyses indicated that A. pleuronectes mainly fed on Bacillariophyta (62%), a small quantity of Chlorophyta (8%), Pyrrophyta (8%), and Cyanophyta (6%). Species of Diploneis, Cyclotella, Pleurosigma, and Navicula played dominant roles in the stomach. The nitrogen and carbon stable isotope ratios were 6.41‰–12.80‰ (average 8.89‰) and –21.45‰ to –17.56‰ (average –19.54‰), respectively. Significant differences (P < 0.01) were observed among seasons in δ¹⁵N, with average values of 9.53‰, 9.80‰, 8.79‰, and 8.34‰ from spring to winter. The analysis of the feeding habits and stable isotopic patterns indicated that A. pleuronectes could be used as isotopic baseline indicators. It is recommended that A. pleuronectes can be used as a common isotopic baseline indicator for the ecology studies in the Beibu Gulf or even in the South China Sea because there is no standard baseline in this marine ecosystem.