Beam　quality　is　a　core　issue　in　the　field　of　terahertz　full-field　imaging.　In　this　paper,　we　present　a　terahertz　spatial　filter　consisting　of　two　mounted　ellipsoidal　silicon　lenses,　which　have　the　same　eccentricity　but　unequal　size,　and　an　opening　aperture　on　a　thin　gold　layer　between　the　lenses.　At　the　frequency　of　2.52　THz,　the　beam　transmissivity　and　Gaussicity　of　the　filtering　system　is　investigated　by　simulation.　COMSOL　Multiphysics　is　used　to　conduct　two-dimensional　simulation　experiments,　aiming　to　find　the　appropriate　aperture　size　and　gold　layer　thickness,　depending　on　the　wavelength　of　incident　light　wave　and　numerical　aperture　of　the　ellipsoidal　silicon　lens.　The　filtering　system　is　capable　of　filtering　the　non-Gaussian　beam　to　a　nearly　fundamental　Gaussian　beam,　and　achieves　a　high　transmissivity.　In　terahertz　full-field　imaging,　the　system　can　not　only　obtain　an　imaging　beam　wtih　uniform　intensity　distribution,　but　also　reduce　the　beam　energy　loss　caused　by　multiple　surfaces.　Besides,　the　lens　system　is　applicable　for　a　wide　terahertz　frequency　range　if　the　wavelength　dependent　part　is　properly　scaled.　©　2022　SPIE.