The magnetic resonant coupling based wireless power transfer utilizes the high-frequency alternating magnetic field coupling fundamentally. Once a metallic foreign body invades, the eddy current effect generated by the foreign body may bring considerable security risks to the system. At present, the detection methods of metallic foreign body in this wireless power transfer system are mainly based on power loss and thermal effect caused by foreign body. Its dominate disadvantages relay mainly on the high delay and complex parameter extraction. Under the background, a new detection system of metallic foreign body location based on three-stage coil is proposed in this paper. The first and second stage coils are composed of two parallel rectangular unit detection coils. The second stages coils are arranged perpendicular to each other to achieve the simple positioning of metallic foreign body in a quarter of the charging range. The third stage coil is composed of four isosceles triangular unit detection coils that just cover the transmitting coil. The locating detection of metallic foreign body within one eighth of the charging range is then realized. Finally, a magnetic resonant coupling based wireless power transfer system with an area of 340 mm×280 mm transmitting coil is applied for verification. It is shown that a square iron block with side length as small as 20 mm can be detected by the system when the threshold voltage is 1.4 V.