The developing of THz fiber would be an important issue for exploring the full potential of THz communication systems. Although many types of fiber including tube fibers, anti-resonant fibers and photonic crystal fibers have been reported, the further applications are blocked due to the comparatively huge size, the lack of flexibility, high transmission loss, etc. The total internal reflection effect provides a promising motivation to design more practical THz fiber, the same as the breakthrough in optical fibers. In this work, we worked on the exploration of advanced polymer composites with high dielectric constant and ultralow transmission loss for designing and fabricating THz fibers. A flexible solid core-cladding structured THz fiber with transmission loss of around 3 dB/m can be obtained. This work may provide the roadmap towards real-practical THz fibers.