Abstract: In this paper, using the largest entrpy spectrum method, the entropy spectrum analysis is made for the obserational data of groundwater dynamic state of many years in several wells in Hebei Province and its neighbouring area. The result shows that the variation of groundwater dynamic state, besides one year and less than one year short period component, showed about three years and ten years predominent period component.This understanding on the period structure can help to correctly recognize the midterm seismic anomalious information.