Abstract: The geomagnetic field model in China offshore area is built for ship's demagnetization, which is overlapped by international geomagnetic reference field and the earth's crust field described by the grid node. The way of modeling and obtaining of the centrosphere field, the earth's crust field and the geomagnetic field for demagnetization are introduced in this paper. Methods of building and using of the inquire search system are introduced in the end.