The fractionation of carbon and nitrogen stable isotopes by benthic fauna in relation to food sources is fundamental to applying stable isotope techniques in trophic ecology research. To determine the isotopic fractionation coefficients (Δ¹³C and Δ¹⁵N) ofSipunculus nudusfor potential food sources in the tidal flats of Beihai, Guangxi, a controlled feeding experiment was conducted. The experiment measured the fractionation coefficients ofS. nudusfor mangrove leaves,Enteromorpha clathrata,Spartina alterniflora, and shrimp feed. Additionally, field sampling was used to calculate the fractionation coefficients for surface sediment organic matter. The results showed significant differences in Δ¹³C and Δ¹⁵N among the four potential food sources. Δ¹³C exhibited a strong positive correlation with the initial δ¹³C difference betweenS. nudusand the food source, fitting the linear equation: △ ¹³C = 0.8618 × (δ ¹³C_i- δ ¹³C_d) + 0.6619 (R²=0.9886). Δ¹⁵N was negatively correlated with the food source’s C/N ratio, described by: Δ¹⁵N = -0.0446 × (C/N) + 1.6391 (R² = 0.9107). For surface sediments, the fractionation coefficients ofS. nudusranged from 4.11‰ to 5.54‰ for Δ¹³C and 6.13‰ to 8.97‰ for Δ¹⁵N, both exceeding commonly referenced values. The δ¹³C, δ¹⁵N, and C/N ratios of the four potential food sources differed significantly from those of tidal flat sediments, suggesting that mangrove andS. alternifloradetritus are not the primary direct sources of organic carbon and nitrogen in adjacent tidal flats. Instead,E. clathrateand discharges from nearby shrimp ponds may contribute more directly to sedimentary organic matter. These findings provide critical insights into the trophic ecology ofS. nudusand its role in the biogeochemical cycling of coastal ecosystems.