Abstract: The lower reach of Shiyang River Basin (LSRB) is located at the transition area from steppe to desert.There are 23 families,73 genus and 123 species of spermatophyte,which are not only the elements of desert and Gobi,but also the steppedesert species.The LSRB has an obvious characteristic of desert vegetation.The structure of synusia is simple,and 74.4% of association constructive species are shrub or semishrub,which composes of shrubbery or semi-shrubbery xerophytic community.The annul herb accounts for a few rate,but makes up “shrubbery-herb association" and acts as an important element of desert vegetation.In the floristic composition,the component of Temperature Zone is dominant,with superior to the species of Mediterranean Sea to Central Asia and their variants,and the second to the species of North Temperate Zone and their variants.The plants centralize in more adaptive habitats,with an obviously clumped distribution in whole area,but the species distribute simply at a small area.The vegetation can be classified into 4 vegetation types,5 vegetation subtypes,13 formation groups,23 formations,and 38 associations.The zonal vegetation distributes circularly with oases,and can be divided into three belts.Such a circular distribution of vegetation in LSRB is quite common in arid zone.

Key words: Combined flocculants, Flocculent performance, Turbidity, Sewage treatment, Ecological safety