Abstract: At present,the main circulatory pattern of Qiongbei compound aquifer system of Hainan(area of 4300km²)is inter-supplying and inter permeating in vertical. The subsidiary circulatory pattern is runoff in horizontal,and the system inter-cepts the groundwater runof to the sea with maximum limitation. Two condit ions are demanded for the sea and the compound aquifer system to exchange water:(1)existing passage for water to be exchanged,and(2)existing potential difference(the difference of location head and pressure head and the gravity potent ial head produced by the different density of salt and fresh water)Since groundwater resources of Qiongbei compound aquifer system has been continually exploited for many years,the level of the hydraulic has already been reduced under the sea level for fifteen to twenty years ago.Up to now no obvious sign of the sea invading has been found.