Piezoelectric　vibration　energy　harvesting　has　attracted　considerable　attention　because　of　its　prospects　in　self-powered　electronic　applications.　There　are　a　many　low-velocity　waters　in　nature,　such　as　rivers,　seas　and　oceans,　which　contain　abundant　hydrokinetic　energy.　In　this　paper,　an　optimal　geometric　piezoelectric　beam　combining　magnetic　excitation　is　identified　and　applied　to　a　vortex-induced　vibration　energy　harvester　(ViVEH)　for　low　velocity　water　flow,　which　is　composed　of　a　continuous　variable-width　piezoelectric　beam　carrying　a　cylindrical　bluff　body.　The　finite　element　simulation　and　experiment　are　first　carried　out　to　study　the　harvesting　characteristics　of　the　designed　variable-width　beam　ViVEH　without　considering　the　magnetic　excitation.　The　influence　of　the　width-ratio　and　flow　velocity　on　the　harvesting　voltage　is　studied　in　detail.　The　optimal　structure,　a　ViVEH　equipped　with　triangular　piezoelectric　beam,　is　then　obtained　by　the　superior　energy　harvesting　performance　for　low　velocity　water　flow.　From　the　experimental　results,　at　a　flow　velocity　of　0.6　m/s,　the　highest　root　mean　square　(RMS)　voltage　and　RMS　voltage　per　unit　area　are　19.9　V　and　0.07　V/mm(2),　respectively.　Furthermore,　magnetic　excitation　is　introduced　to　improve　the　scavenging　performance　of　the　optimal　triangular　beam　ViVEH,　different　polarity　arrangements　are　compared,　and　the　optimal　case,　the　arrangement　of　horizontal　repulsion　and　vertical　attraction　(HR-VA),　is　obtained.　This　case　can　scavenge　the　highest　power　of　173　mu　W　at　a　flow　velocity　of　0.5　m/s,　which　is　increased　by　127%　compared　to　a　conventional　constant-width　beam　ViVEH　with　no　magnetic　excitation.