At Pie-kang area, awing to the overpumping of groundwater, the water level is declined and the water quality is deteriorated. After field observation and study on groundwater quality, the author comes to the following conclusions. 1. For many years, the water level had continued to decline, the lowest one at kou-Hu was 25 meter below mean sea level. It is due to overpumping of groundwater by a lot of fish pond and agriculture use. 2. The electrical conductivity of most water of deep wells was between 300 to 750 μσ/cm, and those the electrical conductivity was more than 750 μσ/cm at the beginning of pumping will descended later on as time went by. 3. By the chemical analysis of water, the ratio of S042-/CI-, bar diagram, the ratio of CI-/(C032- + HC03-), Piper's trilinear diagrams, the rate of SAR and so on, the quality of most water of deep walls was good. The water of poor quality gathered at the area southern Niu-Tiau-Wan drainage channel. It also shows there is no seawater intrusion now. 4. The poor ground water quality in some areas were due to the deterioration of shallow aquifer, which was due to flow backward of seawater. There fore well contruction should avoid shallow aquifer or isolate it. 5. In order to prevent the deterioration of groundwater quality, the better ways are to reduce the pumping amount, prevent flow back of seawater and improve drainage system etc.