In light of the idea of 'functional unit' in combination with 'sequential structure', a THz ultra-high birefringent photonic crystal fiber is proposed by introducing the hierarchical microstructures into the fiber core region. The fiber cladding is composed of wavelength-scale circular air holes arranged in triangular array and the fiber core region consists of subwavelength-scale elliptical air holes that satisfy the equal-differential condition for adjacent layers. Numerical simulation results indicate ultra-high birefringence and low effective material loss is supported in the proposed THz fiber for an operation frequency range of

0.5-1.5 THz. And the highest modal birefringence reaches 4.07×10^-2 at 0.9 THz, corresponding to effective material losses of x- and y-polarization modes are 0.075 cm^-1 and 0.062 cm^-1 , respectively. Moreover, the modal birefringence properties of the proposed THz fiber is investigated in comparison with those of the uniform microstructured fibers of six different sizes. The results show that the proposed hierarchical structure would help to achieve flattened modal birefringence within wide THz operation frequency range, which would be of particular significance for the optimization of communication devices and sensors that could be applied in the field of THz sciences.