Abstract: Investigations of waveguide application to the whole optical cavity of CAEP’s high power THz-FEL device are achieved. The influence of waveguide on the quality of optical cavity is calculated by theoretic analysis and simulations. The waveguide gae size is designed to be 14 mm and 22 mm. In the experiment, we found it was have to get lasing in 1−2 THz. By analysis, we think the reason is that waveguide is made of Titanium and the waveguide wall roughness is unsuitable, which induce large diffraction loss of optical resonator. To reduce the loss, we use a new waveguide made of Cu to replace the old one. Using the new waveguide, lasing and saturation were attained in 1−2 THz in 2019. Then the whole facility counld run stably in 0.7−4.2 THz using the 14 mm Cu wave-guide.