Abstract:Investigations of using the waveguide in the whole optical cavity of CAEP high power THz- Free Electron Laser(FEL) device are carried out. The influences of waveguide on the quality of optical cavity, such as the loss, the gain and output power of THz laser at different frequencies are studied by calculation and theoretical analysis. The results show that the gain of optical cavity can be increased almost by 30%. Furthermore, the influences of two waveguide sizes, 14 mm and 22 mm, are compared. This work will be helpful to the designs of CAEP THz-FEL equipment.