Sea water intrusion once occurred in the western section of the Haicheng District in Beihai, Guangxi. Geochemical processes occurring during sea water intrusion is of concern. An attempt to identify the geochemical processes occurred in the intrusion of seawater in the coastal aquifer in the study area was made by using methods of ionic deltas, ionic ratios, saturation indices and hydrogeological simulation. In genera, dolomitization may easily happen after sea water intrusion, simulation must be done to make sure if it happen. The results by several methods suggest that in addition to a direct mixture of freshwater and seawater, dissolution of K-feldspar, dolomite and chlorite and deposition of calcite and illite were identified in the aquifer. In addition, cation exchange, dolomitization may also occur. Cation exchange took place between the clay and the groundwater and is quite clear as reverse exchange (removal of Na+ from solution).The dissolved quantity of dolomite is positive, this results suggest that dolomite dissolution and dolomitization impossibly happen in this zone.