To study the species composition and morphological characteristics of Siphonosoma in Zhaoshu Island, an onsite investigation was conducted in the tidal flat in Zhaoshu Island in Summer 2018. The environmental adaptability of Siphonosoma in tidal flat were analyzed based on the sediment characteristic and light microscopic observation of tissue biopsies. Three sampling section were selected in the tidal flat to collect and identify the macrobenthos qualitatively and quantitatively, and the sand grain size composition in gut and slices of gut were determined. The results indicated that, There are three species of Siphonosoma, and they are S. rotumanum, S. cumanense and S. funafuti, the species composition was about 4︰2︰1. The three species lived in the same environment and disturbed the same level of 10﹣15 cm. The gut of Siphonosoma was filled with sand, and the small organic particles, diatoms, copepods and plant fibers were found in gut. The large particles in down-gut were more than that in the upward-gut, and the small paticles in down-gut was less than that in the upward-gut. The result indicated that the gut of Siphonosoma has grain refinement effects to sediment particles. The cuticle of introvert retractor muscle and body wall were 59.08 μm and 231.92 μm, respectively. The introvert retractor muscle consisted of 9 000﹣9 500 muscle fibers, and there were some mastoid in the skin. The thicker cuticle and strong introvert retractor muscle were two adaptions of Siphonosoma to the coral reef sediment environment. The numbers of folds and muscle fibers in down-gut were less than that in the upward-gut. This may be correlated to the low content of organic matter in the coral reef sand.