Topological photonics has been extensively investigated due to its extraordinary properties, such as the robustness of immunity to defects, single-mode transmission, and linear dispersion. However, conventional topological insulators in terahertz region are challenging to be compatible with the COMS process. Here, we demonstrate the topological valley spin states in the terahertz transmission line device with -1.5dB ohmic loss and minimal reflection loss $(\sim 0.3\text{dB})$, realizing a high mode conversion efficiency between topological propagation mode and microstrip quasi-TEM mode at a broadband width of $\sim 20\text{GHz} (>\sim$ 15% of the relative bandwidth) which have never been achieved before and have great potential in the application of terahertz on-chip communication applications.