The invention discloses a three-dimensional tomographic imaging system and method. The system comprises a diffraction-free device, a polarizing assembly, a scanning device and a receiver, wherein thediffraction-free device is used for converging incident first light beams into second light beams, and the first light beams are terahertz light; the polarizing assembly is arranged on the light emitting side of the diffraction-free device and used for emitting light in a preset polarization direction in the second light beams to the scanning device; the scanning device is used for reflecting thelight in the preset polarization direction to a tested material in a scanning manner and reflecting third light beams reflected by the tested material to the polarizing assembly; the polarizing assembly is also used for changing the polarization direction of the third light beams and reflecting the third light beams in the changed polarization direction to the receiver; the receiver is arranged onthe reflection side of the polarizing assembly and used for receiving the third light beams reflected by the polarizing assembly so as to construct a three-dimensional image of the tested material according to information of the third light beams. Detection speed of the material and imaging accuracy can be improved.