| when the wireless power transmission system is working, there may be metallic foreign bodies getting into the space between coils. These metallic bodies can produce eddy current effect, which can generate eddy current loss and cause temperature rise, bringing about greater potential safety hazards. The eddy current loss generated can increase the equivalent input impedance of the primary coil, and the higher permeability of magnetic metal can enhance the eddy current loss. Through the finite element simulation, this paper finds out the changing rule of the equivalent input impedance when metal foreign bodies move vertically and horizontally, as well as change materials and material volumes, and further figures out that this change makes the voltage attenuation amplitude of the primary coil under free resonance present a regular concave shape, which is obviously different from that without foreign bodies. Finally, it tests the attenuation curves of metal foreign bodies under different parameters through experimental devices, and proposes a method to detect metal foreign bodies based on the LC free resonance state. The method combines the structural characteristics of the transmitting end to identify metal foreign bodies, which has high accuracy and has no detection blind areas.