The performance of the ABS/Ag-coated terahertz hollow waveguide (HWG) was improved through plasma treatment of the ABS structural tube. The adhesion of the silver (Ag) film to the ABS tube was enhanced from level 5 to level 2 after plasma treatment. The 4.2 mm bore waveguide sample treated with plasma has a more uniform and denser silver film than the untreated sample, which contributes to the reduction of transmission losses from 0.72 to 0.70 dB/m at 0.3 THz and 1.47 to 1.44 dB/m at 0.1 THz, respectively. After 200 hours of hydrothermal aging and 16 cycles of high and low temperature cycling testing, the straight loss of the HWG sample treated with plasma increased by less than 0.1 dB/m, while the untreated sample underwent an increase of more than 1.0 dB/m. The results indicate that the ABS/Ag-coated HWG fabricated by plasma treatment has lower loss, higher reliability and better anti-aging performance compared with the untreated sample. It can be potentially used for establishment of next-generation communication, sensing, and THz imaging systems.