A novel broadband high birefringence terahertz photonic crystal fiber （PCF） with cascaded hexagonal unit porous core based on Topas cycloolefin copolymer is proposed. The full vector finite difference method is used to analyze the guided wave characteristics. The results show that the ultra-high birefringence of 0.096 5 （close to 10^-1）， the ultra-low confining loss of 10^-12 dB/cm and the effective material absorption loss less than 1 cm^-1 are obtained at the frequency of 3.5 THz. In addition， the proposed PCF exhibits nearly zero flat dispersion of ± 0.2 ps/THz/cm in the frequency of 2.25~5 THz. The birefringence value of the proposed PCF is not only the highest among the reported THz polymer fibers， but also the novel porous core structure effectively reduces the transmission loss of THz wave. Moreover， only circular air holes are used in the fiber， which is feasible for engineering fabrication. This work has certain reference value for the future development of THz PCF.