For the difficulty of identifying relay-tracked targets across cameras, a method for re-identifying targets in frequency domain is proposed. By locally averaging, the non-rigid target's Fourier spectrum is approximated by the real sum of its relatively rigid components' spectrum in amplitude. Then, by using the narrowest sampling window to limit the components' changes, the target spectrum's change is limited to a low degree. Finally, the consistency of the amplitude changing rate at every point of the spec...