For a wireless power transfer system with one single planar rectangular pad, a metal object detection (MOD) method based on vertically decoupled coil sets is proposed. The detection coils are combined in a reasonable manner to form a detection coil set, which is decoupled from the transmitting and receiving coils simultaneously. When there is no metal object, the induced voltage of the detection coil set will be close to zero and minimally affected by the offset of the receiving coil. When a metal object falls on the top of the transmitting coil, eddy currents will be generated and induced voltage will appear in the detection coil set. The detection accuracy of the proposed method is not significantly affected by the offset of the receiving coil, and there is no need to reset the detection circuit or adjust the threshold for different offset positions. The parameters of the detection coil are optimized by finite element simulations. Based on a 300 W wireless charging system, MOD experiments were conducted, and results show that the proposed method can accurately identify metal objects even in offset situations. This method solves the problem of a planar detection scheme being affected by the offset of the receiving coil, with high detection sensitivity and great engineering application value.