The　structure　optimization　of　piezoelectric　energy　harvester　aims　at　frequency　reduction,　broadening　bandwidth　and　increasing　output　mainly.　This　paper　employs　a　cantilever　with　cavity　modelled　by　the　finite　element　software　COMSOL　Multiphysics　and　the　effect　of　the　cavity　on　transducer　is　studied.　The　first　order　characteristic　frequency　is　obtained　by　modal　analysis　module　and　the　frequency　response　curve　is　solved　by　frequency　domain　analysis　module.　Comparison　results　show　that　the　introducing　of　cavity　improves　output　voltage　of　energy　harvester　and　reducing　device　nature　frequency.　Moreover,　effect　of　the　cavity　size　parameters　such　as　the　thickness,　length　and　position　on　frequency,　output　and　conversion　rate　of　the　harvester　is　studied,　after　that　the　energy　harvester　structure　is　optimized.