THz Quantum Cascade Laser(QCL) is an ideal solid-state THz source. It is an important part of THz QCL design to investigate the influence of waveguide structures on the properties of lasing light and beam quality. The confinement and loss properties for double-metal and single-metal waveguides of THz QCL are analyzed using finite element method. The dependence of confinement factor，waveguide loss and threshold gain on the waveguide structure, the lasing wavelength and other parameters is presented. The results show that compared with the single-metal waveguides，the double-metal waveguides have better light confinements and lower losses，so they are more suitable for waveguide confinement of active region. Based on the calculated light field distribution in the waveguides，the beam qualities are analyzed using the vector diffraction theory. Far-field beam widths and divergence angles for different waveguide widths are given，which has provided a helpful reference for QCL design from application perspective.