We　demonstrate　a　broadband　near-100%-efficiency　ultrathin　metasurface　operating　at　microwave　and　millimeter-wave　frequencies.　We　develop　and　employ　two　orthogonally　polarized　metallic　gratings　to　form　a　Fabry-Perot　cavity　and　incorporate　a　subwavelength　metallic　double-split-ring　resonator　at　the　center　of　each　unit　cell.　It　allows　arbitrary　amplitude-phase　combinations　with　no　coupling　between　amplitude　and　phase　or　between　transmitted　and　reflected　waves,　leading　to　the　design　of　an　ultrathin　but　highly　efficient　broadband　metasurface　with　multiple　functionalities.　Furthermore,　the　proposed　metasurface　can　generate　diffractive　beams　with　different　orders　and　vortex　beams　with　different　orbital　angular　momentum　(OAM)　modes　in　reflection　and　transmission　spaces　simultaneously.　Both　numerical　and　experimental　results　verify　that　the　proposed　metasurface　has　superior　performance　to　its　counterparts　that　are　based　solely　on　phase　control.　The　proposed　metasurface　presents　a　lightweight,　low-cost,　and　easily　deployable　flat　device　for　microwave　and　millimeter-wave　applications.