Abstract: Thermal power plant water treatment system using the water island construction mode in the water is capable of taking better advantage of the cascading water usage while saving more occupied area and equipment investment. For some coastal thermal power plants that rely on seawater desalination as the industrial water for the whole plant, it is of greater significance to apply the construction mode of the water island. Take a coastal thermal power plant as an example, this paper introduces the idea of water island system design for this type of thermal power plant water treatment system, analyzes the layout and decentralized arrangement in terms of area and economic benefits, then points out the items that more attention needs to be given in the design of water island system. The analysis shows that the centralized deployment of water islands in coastal thermal power plants occupies 20% of the floor area less than conventional decentralized arrangements, and it can also save the investment on a large amount of public facilities. Therefore the target to downsize staff and improve economic efficiency is reached while the level of water regulation is enhanced with significant economic and environmental benefits.