Abstract: Effects of electron beam energy spread, normalized emittance and bunch length on the gains of free-electron laser terahertz source were discussed, and further considering waveguide restriction of the vacuum chamber in undulator, longitude and transverse coupling between terahertz radiation fields and the electron beams were analyzed. The results show that waveguide effects improve transverse coupling, and as the waveguide effects slowing down the propagation velocity of terahertz wave will help to improve the longitudinal coupling. The saturation procedures of the terahertz radiation field were studied, and the electron bunch lengths were also optimized in waveguide case.