When the metal object is intruded into the wireless power transfer (WPT) system, it will not only affect the transmission efficiency of the system, but also lead to the occurrence of fire and other safety accidents. An active detection scheme for metallic object based on impedance variation is proposed in this paper. By adding a detection coil array and an excitation source on the surface of the transmitting coil of the radio energy transmission system, the detection problem is solved when the passive detection scheme cannot realize in the case of power loss. The structure of the detection coil is designed, compared with the traditional detection coil scheme, and the blind spot problem of the traditional detection scheme is solved. The selection principle of resonance frequency in the detection coil loop is discussed, and the deviation frequency is selected to improve the detection sensitivity. The detection effect of detection coin scheme on different types of metal objects was studied by setting up the finite element simulation model. The experimental results show that the designed detection scheme can effectively detect the location of metal foreign bodies when they are mixed with the radio energy transmission system.