At terahertz frequencies, three types of Bessel resonators, namely, a stable Bessel-Gauss resonator, a Bessel resonator, and an unstable Bessel-Gauss resonator, are built based on quasi-optical theory and technology. To strictly analyze the diffraction field distribution characteristics of the cavity, the boundary element method is used to calculate the iterative dyadic Green′s functions (IDGF), whose algorithm is then used to calculate the three-dimensional self-reconstructing model of the cavity. The zero order model and high order model of three types of Bessel resonators are compared and the results are given. The Bessel beam or Bessel-Gauss beam generated are expected be used in such fields as communications, measurement and imaging in quasi-optical or terahertz spectrum.