The invention is applicable to the field of waveguide technology, and provides a low-loss small mode field terahertz waveguide. The terahertz waveguide is in a direction perpendicular to a propagation direction of the terahertz waveguide, is composed from outside to inside of a metal layer and a gap layer, and further consists of a dielectric layer inserted into the gap layer. According to the invention, the dielectric layer is inserted into the gap layer of the metal layer, and the energy loss can be less as the energy loss is mainly concentrated in the metal layer while the dielectric layer inserted in the gap layer of the metal layer can enable transmission of less energy in the metal layer; in addition, as the metal layer has a pitch of sub-wavelength order, the terahertz waveguide can conduct THz waves having the characteristics of low loss and sub-wavelength order mode width.