The wireless power transfer technology has been gradually applied to various fields of production and life. When there are metal foreign objects between or around the power transmitter and the power receiver of the magnetic coupling wireless power transfer system, the transmission performance of the system is greatly affected, and even a safety hazard may occur. Aiming at the S/S type magnetic coupling wireless power transfer (MC-WPT) system, this paper proposes a non-ferromagnetic metal foreign object detection method based on equivalent quality factor, and gives the definition of equivalent quality factor. The equivalent quality factor model of S/S type MC-WPT system under ferromagnetic metal foreign objects, the principle and method of detecting metal foreign objects are expounded. Finally, the feasibility and effectiveness of the proposed metal foreign object detection method are verified by simulation and experiment.