The　importance　of　the　terahertz　(THz)　region　in　wireless　communication　is　increasing.　THz　tunable　filters　are　key　components　for　next-generation　communication　systems.　In　this　article,　we　have　designed　a　THz　narrowband　bandpass　filter　based　on　a　double-layer　frequency　selective　surface　(FSS),　which　is　composed　of　two　X-shaped　FSSs　cascaded　together　with　different　geometric　parameters.　The　center　frequency　of　the　double-layer　structure　is　located　between　the　center　frequencies　of　the　two　single-layer　FSSs.　Based　on　the　principle　of　inverse　design,　we　employed　intelligent　algorithms　to　automatically　optimize　the　structural　parameters　of　the　single-layer　FSS.　We　simulated　the　transmission　spectra　of　both　single-layer　and　double-layer　FSSs　using　finite-difference　time-domain.　Subsequently,　FSSs　were　fabricated　using　femtosecond　laser　micromachining　and　characterized　using　time-domain　THz　spectroscopy.　The　experimental　results　are　highly　consistent　with　simulations.　The　designed　double-layer　filter　exhibits　a　high　Q　factor　and　tunability　of　the　center　frequency　by　changing　the　spacing　between　the　two　FSS　layers.　Finally,　based　on　an　equivalent　circuit　model,　we　analyzed　the　transmission　phenomenon　of　the　nonequivalent　double-layer　FSS　structure　and　attributed　it　to　the　resonances　of　equivalent　capacitance　and　inductance　between　the　FSS　layers　at　different　spacings.　This　approach　can　be　applied　to　design　tunable　filters　for　other　frequency　bands.　©　2011-2012　IEEE.