Abstract: A mathematical model for seawater intrusion in aquifers is presented; which is capable of handling both confined and phreatic aquifers and mainly contains two partial differential equations. The first equation is used to describe flow of variable-density fluid (mixture of fresh water and seawater), and the second equation is used to describe the transport of dissolved salt. The model is demonstrated by using the three-dimensional observation data of seawater intrusion in Longkou, Shandong Province.