A fabricated microstructured polymer optical fiber (MPOF), with porous core and porous cladding consisting of a hexagonal array of circular air-holes, is experimentally investigated via a terahertz (THz) time-domain spectroscopy (TDS) setup. The fiber features a low propagation loss ranged from 0.0180 to 0.0345 cm−1, in the frequency regime of 0.3–0.5 THz with diameter of 3 mm. Meanwhile, the dispersion and fabrication deformation are also discussed. Experimental results show there is a flat and near-zero dispersion band in the range of 0.39 to 0.45 THz with a value of −0.285 ± 0.02 ps/THz/cm The simulation and experimental results of transmission loss and dispersion are in good agreement within 0.3–0.5 THz. There are a few deviations regarding air porosity between theoretical and actual structure, and the absolute variation of air hole fraction in core and cladding is 0.485% and 1.657%, respectively. The proposed MPOF is potentially useful for efficient and convenient transmission of broadband THz radiation.