Abstract: The effect on the antenna radiation characteristics of the platform can not be ignored. But some radiation problems of antenna mounted on electrically large platform can not be calculated due to the limitation of the physical memory capacity. To solve the above problems, OutofCore solving method using hard disk instead of memory is used. In order to accelerate the solving speed, one block Gauss elimination method is presented. The matrices are read and written by blocks concentratively, so the time of the data reading/writing on the hard disk can be greatly reduced. The radiation patterns of a monopole mounted on a metal platform and a vehicular antenna model are calculated respectively. The results show that, the problems of antenna mounted on electrically large platform can be effectively solved by the method proposed in this paper, the solving time is only 5.9% more than the InCore solving method's, and the calculation accuracy is not lost.