Abstract：On the basis of the FFT spectrum and wavelet transform theory, the changing characteristics of electromagnetic wave frequence spectran of Dongsanqi station in Beijing before the Zhangbei earthquake are studied.The results are as follows.1) Anomalies did exist before the Zhangbei earthquake. 2) Anomalous signals of the electric field are more significant than those of the magnetic field, and the NS component is stronger than the EW component.3) Anomalies appeared concentrately on Dec. 31,1997 and Jan. 1 , 1998 and there were significant phase changes that it might reflect the different stage characteristics of earthquake preparation.
