Stable isotope and laboratory(feeding selectivity & detritus size-fraction)experiments were used to investigate feeding behavior and ecological function of the gastropod, Turbo cornutus Solander(T.cornutus), living on the seaweed bed during the summer, 2013.Stable isotope analysis demonstrated that T.cornutus is herbivore and primary consumers(trophic level is 2.12)in seaweed bed, and prefers seaweed(>94.5%)to other potential food resources(phytoplankton, POM and SOM).Laboratory experiments revealed that T.cornutus could feed(>0.1 g individual^-1day^-1, wet weight)11 different seaweeds(including the dominant seaweed species of Gouqi Island: Sargassum horneri and Grateloupia livida).Detritus size-fraction experiments provided evidence that T.cornutus can digest seaweed, then excrete detritus(0.016-0.094)g/d into small size(dominant detritus size is 1Φ:>250 μm), these change of matter size-fractionate caused by T.cornutus grazing behavior could make contribution to detritus in seaweed bed.Stable isotope experiment, combined with laboratory(feeding selectivity & detritus size-fraction)experiments suggest T.cornutus plays an important role in connecting grazing food chain and detritus food chain in food web of seaweed bed, Gouqi Island.