Abstract: A new frustum Bessel resonator is designed in this paper. The principle for generating Bessel beam is analyzed by Hankel wave theory, and the geometric optics theory is employed to evaluate the extent of Bessel beam on the optical axis. In order to overcome the problem of beam divergence caused by the size of resonant cavity, one can adjust the propagation direction to solve it. An iterative equation based on dyadic Green′s function is used to calculate 3D representation model of cavity. In the Terahertz frequency, the numerical simulations have been carried out to demonstrate the focusing conclusion of Bessel beam through comparing the field distribution and its full width half maximum (FWHM) on the face mirror. The created Bessel beams have potentially extensive applications.