During wet season and dry season, the groundwater quality analysis results of one engineering site in Nanning indicated that the causticity of groundwater mainly included acid causticity (pH value), calcareous causticity (corrosive CO2) and leaching causticity (HCO3-). Though the twice analysis results of water causticity are both moderately corrosion, the types of decomposed corrosion are different. It indicated the uncertainty and changeability of decomposed corrosion. In actual projects, some special ingredients in groundwater, the site natural environment and hydrologic geologic conditions should be considered together to define the groundwater’s causticity. Moreover, the change of site hydrologic geologic conditions caused by the construction should be considered, because this change can also influence the groundwater’s causticity. In addition, the groundwater’s causticity will influence the selection of cement types and the type of cement will influence the type of foundation, then all these will influence the construction cost finally. So, the change of groundwater’s causticity caused by the site hydrologic geologic conditions should be considered at engineering design time.