Piezoelectric　energy　harvesters　tend　to　convert　energy　from　ambient　vibration　to　electricity,　which　can　gain　a　higher　output　voltage　at　resonance　region.　How　to　obtain　a　larger　working　bandwidth　is　urgent　to　explore　at　present.　In　this　paper,　a　cantilever　plate　energy　harvester　is　designed,　with　two　boxes　added　at　two　free　edges　along　the　fixed-free　direction　and　one　rolling　ball　placed　in　each　boxes　respectively.　The　natural　frequency　of　the　structure　can　be　adjusted　by　changing　the　centroid　of　the　structure,　which　is　caused　by　the　balls　rolling　in　the　box.　The　range　of　adjustment　that　contains　from　the　maximum　(upper　limit)　to　minimum　(lower　limit)　frequency,　is　defined　as　the　theoretical　frequency　band.　The　mode　function　of　tunable　cantilever　plate　is　given　according　to　deflection　curve.　Considering　the　rigid　body　motion　of　boxes　and　the　composite　motion　of　the　balls,　the　Hamilton　principle　and　the　Hertz　contact　theory　are　taken　into　account　to　derive　the　vibration　equations　of　the　system.　The　experiment　is　carried　out　to　verify　the　correctness　of　the　theoretical　mode　and　the　dynamic　equations.　The　responses　of　the　structure　under　different　excitation　frequencies　are　calculated　by　using　MATLAB　software　and　verified　by　frequency　sweep　experiment.　The　results　show　that　the　working　bandwidth　of　proposed　structure　under　different　balls　initial　positions　is　wider　than　with　that　of　beam　structure　in　resonance,　thus　proving　the　better　tuning　ability　of　the　designed　system.　In　addition,　the　output　of　the　energy　harvester　under　different　external　excitation　acceleration　is　given.　The　effects　of　different　length　of　box　on　the　working　bandwidth　and　output　power　density　are　studied.　The　relationship　between　conversion　rate　and　external　resistance　is　proposed　under　different　boxes　length.　A　high　performance　energy　harvester　with　wider　operating　frequency　range　is　designed.