In　this　paper,　a　tunable　cantilever　plate　structure　is　presented,　in　which　a　box　is　attached　to　each　corner　of　its　free　sides　and　a　ball　is　placed　in　each　box.　Change　of　the　balls　position　makes　natural　frequency　of　the　structure　variable,　which　expands　working　bandwidth　of　the　structure　to　accommodate　the　complex　ambient　vibration.　The　modal　analysis　of　the　tunable　cantilever　plate　is　carried　out　by　the　finite　element　software-ABAQUS.　Changing　the　balls　position,　it　is　found　that　there　are　maximum　and　minimum　values　in　the　natural　frequency　of　the　self-tuning　structure,　which　forms　a　resonance　bandwidth.　Moreover,　there　are　multiple　position　combinations　of　two　balls　at　the　same　natural　frequency　in　the　range　of　resonance　band.　The　paper　studies　the　influence　of　mass　and　geometrical　dimensions　of　boxes　and　balls　on　the　natural　frequency　and　bandwidth.　While　ensuring　strength　and　stiffness　of　the　structure,　optimal　material　and　size　parameters　are　selected　to　improve　the　resonant　bandwidth.　This　provides　a　theoretical　basis　for　applying　the　tunable　structure　to　piezoelectric　energy　harvester.