We　present　a　broadband　tunable　polarization　converter　in　terahertz　(THz)　frequency　based　on　a　hybrid　metamaterial　comprising　I-shaped　metallic　resonators　and　double　layer　graphene　sheets.　The　proposed　device　can　dynamically　switch　its　functionalities　among　linear-to-linear,　linear-to-circular,　and　linear-to-elliptical　polarization　conversion　in　broadband　by　electrically　controlling　the　Fermi　energy　of　the　graphene　sheets　without　reoptimizing　and　refabricating　the　structures.　This　device　concept　offers　a　further　step　in　developing　broadband　tunable　polarizers　and　polarization　switchers　and　has　potentially　applications　in　THz　communications,　sensing　and　spectroscopy.