This paper aims to understand the carbon and nitrogen stable isotope characteristics of Rapana venosa across various sizes, in order to gain insights into the nutrient dynamics within a typical bay and island water ecosystem. A total of 160 samples were collected through trawling and diving activities conducted in the Jiaozhou Bay and Changshan Archipelago offshore regions during the spring and summer of 2022. In this study, we conducted an analysis of the δ¹³C and δ¹⁵N isotopic values of R. venosa individuals, taking into account variations in body length and weight. Additionally, we calculated the core niche breadth and other related parameters for each specific research area. The findings revealed that the δ¹³C and δ¹⁵N values of R. venosa exhibited a range of -22.12‰ to -16.63‰ and 8.77‰ to 13.48‰, respectively. The mean values for these isotopic ratios were determined to be -19.74‰ to -17.42‰ and 9.64‰ to 12.81‰, respectively. The mean trophic level of R. venosa varied between 2.63 and 3.57 across different size classes. In spring, a notable negative correlation was observed between the δ¹³C values and the body length and weight of R. venosa collected from Jiaozhou Bay, whereas a significant positive correlation was found in summer. In contrast, negligible fluctuations were detected in the samples obtained from the offshore regions of the Changshan Archipelago. The findings indicate variations in food diversity across different seasons and geographic areas. The variations in δ¹³C and δ¹⁵N were more conspicuous within Jiaozhou Bay. In addition, a noteworthy positive association was observed between the δ¹⁵N values and trophic level, as well as body length and weight, in both regions under investigation. This correlation can primarily be attributed to the divergent feeding preferences of R. venosa, whereby larger individuals tend to consume organisms from higher trophic levels, resulting in an elevated trophic level. Furthermore, the R. venosa exhibited a higher trophic level in Jiaozhou Bay compared to offshore in Changshan Archipelago, indicating the extensive food sources and more abundant enrichment pathway of ¹⁵N. The study elucidated the discernible variations in stable isotope trophic niches across different regions and seasons, as evidenced by the evaluation indexes encompassing total area (TA), δ¹⁵N range, δ¹³C range, and corrected standard ellipse area. These findings underscore the impact of geographical location and food source on the isotopic compositions of δ¹³C and δ¹⁵N in R. venosa. Consequently, this research furnishes fundamental data support for the establishment of food webs in coastal ecosystems, while also offering scientific backing for regional biological resource management and ecosystem restoration.