Whether seawater or brackish water desalination, water pre treatment is the key to ensure long term stable operation of reverse osmosis system .When making seawater pre treatment should be fully considered: in sea water there have large number of micro-organism , bacteria and algae， Bacteria and algae breeding, and the growth of microorganisms in water not only can bring a lot of trouble to get water facilities, and will directly affect the normal operation of desalination equipment and process piping .Periodic flood tide, the sea water with a large number of sediment, the turbidity have bigger change ,that easy to cause the seawater pretreatment system work is not stable.