Water treatment feed water exists in various forms can lead to reverse osmosis membrane surface contamination of substances, such as hydrated metal oxides, calcium-containing precipitates, organic matter and biological and so on. The purpose of setting up the pretreatment device of the membrane system is to minimize the membrane surface contamination, through the installation of a suitable pretreatment system, select the appropriate operating conditions (such as water production flow, operating pressure, water recovery rate, etc.), can achieve this purpose. Pretreatment systems cannot completely remove the substances that cause contamination of reverse osmosis membranes, and after proper pretreatment, there is still contamination from colloidal and particulate matter in the water supply. Moreover, in the desalination process, with the increase of salt concentration in the membrane module, there will be some substances precipitated from the water on the surface of the membrane and form a scale layer, covering the surface of the RO membrane.