Abstract: The geomagnetic Z-component observation data from the Gaochun,Liyang,Gaoyou,Lianyungang,Suqian and Haian seismic station are analyzed using finite difference method.Then,the extremal value of the observation data is studied using the method of loading-unloading response ratio.The geomagnetic low-point displacement is also analyzed using minimum value time of the geomagnetic Z-component.The result shows that geomagnetic filed at Gaochun seismic station changes obviously before moderate-strong earthquake.