Human body back-scattering inspection systems and methods are disclosed. In the invention, X-rays modulated by the flying-spot forming unit having spirally distributed flying-spots have a distribution having alternating peaks and valleys on the irradiated surface. In this way, scanning starting times can be precisely controlled to cause two devices to have scanning starting times that are different by a half of a cycle. That is, the beams outputted from one device are at maximum when the beams outputted from the other device are at minimum. In other words, even if the ray source of one device emits rays, it will not significantly affect imaging result of the other device. In such way, the two devices may emit rays and perform scanning at the same time, and thus the total scanning time is reduced.