The invention relates to a hidden object detecting system and a detecting method thereof. The detecting system comprises a signal generating module, a signal emitting module, a control module, a signal receiving module, a signal processing module, an image input module, an image analysis module and a display module, wherein the control module controls the signal generating module to generate millimeter waves; the millimeter waves are spread to a target region through a space, so that a scattering field is generated around hidden objects in the target region; the signal receiving module detects the scattering field; in addition, received echo signals are transmitted to the signal processing module through the control module; the signal processing module performs two-dimensional or three-dimensional image reconstitution on the received echo signals to obtain a change distribution image of dielectric constants inside and around the hidden objects; the distribution image is transmitted to the image analysis module through the image input module; the image analysis module performs denoising and local feature extraction processing on the reconstructed distribution image to obtain a hidden object image; the hidden object image is transmitted to the display module for displaying.