In this paper some key technologies of an optical cavity for a compact Terahertz(THz) Source Based on Free Electron Lasers(FEL) were studied. Firstly, the theory of THz radiation was analyzed by some calculations. The high quality electronic bunch beams with energy 5.6-9.9MeV can produce THz radiation with frequency 1-10THz. Secondly, the configuration of the optical cavity was calculated and studied, and some conditions were discussed in order to obtain a single resonance mode. Thirdly, The loss caused by the diffraction and the coupling aperture was calculated. Finally, an idea of designing an optical cavity and some important data were given.