Abstract: By applying the propagation theory of electromagnetic field,in conjunction with the measured data,the existence of dynamic characteristics of natural alternating electromagnetic field is analyzed,and its formation mechanism and basic characteristics are clarified.Then,the theory basis is supplied for distinguishing and determining the anomaly information of natural alternating electromagnetic field,and for improving the reliability of interpretative result.This study has guiding significance in solving geology problem such as underground water-bearing formation,Karst,dam seepage,and etc.