Abstract: A kind of porous fiber for low-loss Terahertz wave guiding was designed based on Topas cyclic olefin copolymer(Topas COC). Topas COC is a kind of novel Terahertz low-loss material. Terahertz wave was well confined in a series of subwavelength air holes, which were arranged in base material with triangular lattice. Propagation properties of the designed porous fiber were investigated by using the full-vector finite element method. The results indicated that the designed Topas porous fiber had low loss and low dispersion properties within Terahertz frequency. The loss was less than 0.2 cm-1 from 0.4 THz to 1.5 THz, and the dispersion was 1.80.3 ps/THz/cm within 0.48-1.5 THz. The designed fiber structure was relatively simple and feasible for fabrication. It can be fabricated by the thermo-drawing techniques of polymer preforms invented by our group. The results provided theoretical references for fabrication of Topas Terahertz low-loss and flexible fiber waveguides.