Problems of calculating magnetic force by existing method are pointed out. The calculation formulas of magnetic force and magnetic moment acting on the medium are derived，which can be used to analyze the deformation and movement trend of the medium under the action of magnetic field，and examples are given. In this paper，the Maxwell stress formula is further improved. To some extent，the formulas in this paper make up for the deficiency of virtual displacement method，and provide theoretical verification for Faraday’s point of view and equivalent magnetic charge method.