Abstract: In this paper, the authors try to discuss how to study the variation patterns of the earth main magnetic field by using "Chinese Geomagnet Chart" to infer the structural microdynamic state in the ten year period in the future and to study the main area of strong seismic activity and its variation, The variation of main magnetic field may be related to the reaction of crustal blocks or to the matter movement in the earth.B the main magnetic field, the reaction mode dynamic state and strength of blocks may be inferred. The variation patterns of magnetic field have a certain relation with structttral activity and the main area of strong earthquakes in time and space.