Phononic　crystal　could　be　used　in　the　vibration　energy　harvesting　devices　as　its　capacity　to　modulate　elastic　wave　propagation.　This　paper　presents　a　piezoelectric　phononic　crystal　cantilever　beam　with　periodic　variable　thicknesses　for　vibration　energy　harvesting.　The　transfer　matrix　method　(TMM)　is　introduced　to　analyze　the　natural　frequencies　and　the　band　structure　of　the　phononic　crystal　beam.　The　influences　of　structural　parameters,　such　as　thicknesses-ratio　and　lengths-ratio,　on　the　natural　frequencies　and　first　band　gap　of　the　beam　are　investigated　in　detail.　The　results　of　first　two　band　gap　ranges　obtained　via　the　TMM　are　validated　by　means　of　the　finite　element　method　(FEM)　simulation.　Moreover,　the　electromechanical　coupling　responses　of　the　phononic　crystal　beam　are　explored　by　using　the　FEM　simulation.　The　results　demonstrate　that　the　piezoelectric　phononic　crystal　beam　has　more　efficient　voltage　output　in　the　first　band　gap　comparing　with　the　uniform　piezoelectric　cantilever　beam.　In　conclusion,　the　proposed　piezoelectric　phononic　crystal　beam　can　extend　the　bandwidth　of　the　energy　harvester　in　low-frequency　vibrations.