RO membrane of reverse osmosis equipment is used to remove most of the salt in water by making use of the water permeability and salt impermeability of semi permeable membrane. Pressurize the raw water side of RO to make some pure water in the raw water penetrate the membrane in a direction perpendicular to the membrane, salt and colloidal substances in the water concentrate on the membrane surface, and the remaining raw water carries away the concentrated substances in a direction parallel to the membrane. There is only a small amount of salt in the permeated water, and the purpose of desalination is achieved by collecting the permeated water.