Sebastiscus marmoratus was the main economic fish in Gouqi Island offshore waters of north-east Zhoushan Islands, which played an important part in local fishery. This research used carbon and nitrogen stable isotope technology to analyze the feeding habit of S. marmoratus in different seasons and developmental stages and evaluated the nutrition contribution from food organism such as zooplankton, amphipoda, shrimp, crabs and small fishes. It showed that in the four seasons of Gouqi Island offshore waters, the δ¹³C value of S. marmoratus was between -18.2‰ and -14.7‰, of which the average value was -15.6‰; the δ¹⁵N value was between 3.3‰ and 9.9‰, of which the average value was 7.9‰. One-Way ANOVA showed that the δ¹³C value and that of δ¹⁵N were insignificantly different between seasons. This indicated only a slight difference in the feeding habit between different seasons. A linear-regression analysis was carried out, which analyzed the whole body length and the carbon and nitrogen stable isotope ratios and showed the δ¹³C and δ¹⁵N values changed with the body length significantly, which indicated the differences of the feeding habits of different developmental stages were very significant. Cluster analysis divided S. marmoratus with different body length into three groups:one group with the length of 0 to 19 mm, another group 20 to 79 mm and 80 to 109 mm and the third one 110 to 199 mm. ANOSIM analysis showed the differences between the three groups were quite significant. The δ¹³C value of S. marmoratus was close to that of the food organism such as shrimp, crabs and small fishes. When analyzing the nutrition contribution of food organism with IsoSource Software, the research found Caprellidae's average nutrition contribution to S. marmoratus was the greatest, i.e. 22.3%, and that of the fishes, shrimp and crabs was all above 12%. Based on the δ¹⁵N value of S. marmoratus of Gouqi Island offshore waters, the average trophic level was 3.18, indicating that the S. marmoratus was of low-level carnivores feeding on Caprellidae. The research found that S. marmoratus was benthos feeders rather than zooplankton feeders and Caprellidae, fish, shrimp and crabs etc. were the main food source.