With the rapid development of high water-consumption agriculture, thedramatic increase of urban population, and a sustained drought, the ground water is always highly over-exploited in Tanghai County. This is the most important reason to cause salt water or seawater intrusion. According to the change of concentration of Cl- in deep ground water, of depth of salt water roof, and thedistributionofindustryandagriculture in the whole county, the county is divided into highly sensitive, relatively sensitive, sensitive, less sensitive areas to salt water (seawater) intrusion. The prevention measures to these areas are proposed.