Seawater intrusion exists a significant impact on variation of the groundwater and ecological landscape in the islands and coastal zone, which is one of the important geological disasters of the regions. Due to the influence of the fresh groundwater supply and other variable factors in the different time intervals and spaces, seawater intrusion areas have the characteristics of temporal and spatial variation. Taking the southeast of the Donghai Island as the main research target, this paper used the method of piper diagram to analyze the measured data from December 2012 to August 2014, divided the grade of seawater intrusion according to the content of HCO3 - and Cl- , and studied the seawater intrusion in different periods. The analysis show that the Donghai Island underwent many processes of seawater intrusion, of which the percentages are 56.7%, 51.1%, 56.4%, 42.7%, 36.5%, 76.9%, 66.8% and 75.8% respectively.