Polarization　conversion　is　an　important　technique　for　converting　the　polarization　state　of　electromagnetic　waves.　Based　on　the　Fabry-P　&　eacute;rot　(F-P)　resonance　enhancement　in　a　multilayer　metasurface　structure,　the　reverse　design　of　a　broadband,　highly-efficient　and　adjustable　terahertz　polarization　converter　using　a　multi-objective　particle　swarm　optimization　algorithm　is　demonstrated.　The　device　is　composed　of　three　layers　of　metasurfaces　cascaded　together.　The　outer　two　layers　are　a　pair　of　orthogonal　grating　structures,　the　middle　layer　is　a　sawtooth　grating　structure　with　a　45　degrees　angle　to　both　sides　of　the　grating,　and　the　medium　between　the　layers　is　air.　Single-layer　metasurfaces　were　prepared　using　femtosecond　laser　micro-machining,　assembled　through　a　cage　structure,　and　the　interlayer　spacing　was　adjusted　using　two　precision　translation　stages.　The　device　was　characterized　using　time-domain　terahertz　spectroscopy,　and　the　experimental　results　were　highly　consistent　with　simulations,　with　a　conversion　efficiency　of　over　90%.　In　addition,　the　operating　frequency　of　the　device　can　be　changed　by　adjusting　the　interlayer　spacing.　The　F-P　interference　effect　in　the　triple-layer　structure　plays　a　major　role　in　the　excellent　characteristics　of　this　device.　The　device　structure　can　be　used　in　the　design　of　other　adjustable/reconfigurable　optical　devices.