a liquid-crystal-filled polymer photonic crystal fiber is designed and numerically analysised for terahertz wave guiding. bandgap-guiding terahertz fiber is obtained by infiltrating the cladding air holes of index guiding topas photonic crystal fiber with liquid crystal 5cb. structural parameter dependence and thermal tunability of the photonic bandgaps, mode properties and confinement losses of the designed fiber are investigated by using the finite element method. the bandgaps are formed based on antiresonances of the individual liquid crystal inclusions, so the positions of bandgaps depend strongly on the cladding hole diameter and weakly on the lattice constant. bandgaps and the positions of the confinement loss minimum or peaks of the transmission spectra shift toward lower frequency as temperature increased from 25 °c to 34 °c due to the positive dno/dt of 5cb. average thermal tuning sensitivity of -30 ghz/°c is achieved for the designed fiber. at the central frequency of the transmission band, high power transmission coefficient and thus low splicing loss between the aligned liquid-crystal-filled polymer photonic crystal fiber and the unfilled section is obtained. our results provide theoretical references for applications of liquid-crystal photonic crystal fiber in sensing and tunable fiber-optic devices in terahertz frequencies. © 2017 spie.