Based on the theory of near-field scanning detection, a scattering terahertz scanning near-field optical microscopy (THz s-SNOM) system is designed and built. The system achieves nanoscale spatial resolution, breaking the diffraction limit. Using a frequency multiplier module with an operating frequency of 0.1-0.3 THz as the emission source, the THz beam is focused onto the tip of an atomic force microscope (AFM) probe. The tip of the needle acts as a nano-light source, interacting with the sample. After the evanescent waves are converted into radiation waves, they are collected by the parabolic mirrors and imaged point by point in the far field. The topography image of the sample surface and the near-field amplitude map without background can be obtained simultaneously. The resolution of the system depends on the tip radius of the AFM probe, which is independent on the wavelength of the beam used. The near-field image obtained by scanning different samples shows that the resolution is less than 60 nm and spatial resolution achieves λ/26 000.