A sub-terahertz on-chip transmission line based on spoof surface plasmon polaritons (SSPPs) is proposed, and its structure is designed and processed by InP technology. In order to reduce the area occupied by the SSPPs structure with the conventional rectangular grooves, a folded SSPPs structure is proposed. With the same asymptotic frequency, the area occupied by the proposed structure is reduced by 57.2%. In addition, the efficient excitation of the folded SSPPs transmission line is realized by the grounded coplanar waveguide (GCPW) and a gradient transition structure. The SSPPs transmission line is measured by a terahertz on-chip test system. Test results show that the insertion loss of the SSPPs transmission line is less than 3 dB/mm with the frequency band ranging from 110 to 170 GHz, and the return loss is better than 12.5 dB, which is in good agreement with the simulated ones.