Theory of Fresnel scale diffraction and imaging rule of optical mode were used to investigate the resonator with internal lens and basic structures of equivalent resonator are introduced. Calculation and analysis show that a parallel-plane resonator with internal lens may be equivalent to the plane-spheric resonator. l.t is a thermally stable resonator if the alignment of the nsonator mirrors and lens satisfies the formulas derived in this paper. Properties of mode selection of this type of resonator and some design examples are discussed too.