A prototype of optically-pumped terahertz (THz) gas laser (OPTL) with metallic waveguide was designed and manufactured. The output power of THz laser radiation is 150 mW, pumping by a 44 W, 9P36 (9.69 μm) cw CO2 laser. The THz laser medium is CH3OH vapour, and the pressure is 30 Pa. The photon conversion efficiency is 8.4%. The relationships of THz laser power versus working gas pressure and pump power are given. The stability of the output power in ten minutes was measured. The variation of THz laser power was measured by adjusting the length of the THz laser cavity precisely through a piezoelectric translator. The characteristics of the longitudinal modes in the metallic waveguide THz laser are discussed. The experimental work and results are helpful for further researches on the compact folded all-metal waveguide OPTL.