Abstract: The beach sand is the material base of a sand island. It is not only the home for sea animals, but also the evidence of the forming process of a sand island. Coastal dunes may act as barriers to protect vegetation from the damage by wave and surge. Both of them have important research significance for protection of a sand island. Based on the field survey on beach sands, coastal dunes and sand islands and vegetation on the Qilianyu Island in the scientific expedition 2019 in addition to the interpretation of satellite remote sensing data, this paper discussed the geological features, morphological characteristics, and geographic positions of the beach sands, coastal dunes and vegetation as well as their relationship to the development of the sand island. It is revealed that the formation of beach-coastal dunes is not only determined by the dynamic actions of waves, tides and storm surges, but also related to the process of sediment transportation and sedimentation. With regard to the formation of sand island, vegetation plays critical roles. It starts from a pattern of spots in the beginning and then gradually expands to the whole supratidal area, and moves from coastal dunes to the center of island, and finally occupies the whole island. In the process, plant colonization plays a key role in sand fixation and sand island formation. Upon the observations mentioned above, a genetic model of beach sand-beach rock-vegetation-sand island in Qilianyu Island is proposed.