Bistability　has　been　proven　beneficial　for　vibration　energy　harvesting.　However,　previous　bistable　harvesters　are　usually　cumbersome　in　structure　and　are　not　necessarily　capable　of　low-frequency　operation.　To　resolve　this　issue,　this　paper　proposes　a　compact　two-degree-of-freedom　(2DOF)　bistable　piezoelectric　energy　harvester　with　simple　structure　by　using　an　inverted　piezoelectric　cantilever　beam　elastically　coupled　with　a　swinging　mass-bar.　The　swinging　mass-bar　possesses　bistable　property　due　to　the　combined　effect　of　the　gravity　and　the　elastic　joint.　It　is　revealed　that,　under　the　inter-well　periodic　motion　pattern　which　has　large　swinging　amplitude,　the　swinging　mass-bar　can　exert　large　force　and　moment　on　the　piezoelectric　cantilever　beam,　thereby　generating　large　electrical　output　in　this　process.　Moreover,　the　inter-well　periodic　swinging　motion　can　occur　in　a　very　broad　low-frequency　region,　enabling　broadband　low-frequency　energy　harvesting.　An　experimental　prototype　is　tested　under　harmonic　excitation　and　sine　frequency　sweeping　excitation;　high　electrical　output　is　gained　in　the　frequency　range　of　2　Hz　to　12.6　Hz　with　a　peak　power　of　3.558mW　and　a　normalized　power　density　of　19.52mW/(g(2)&　BULL;cm(3)),　which　validates　the　broadband　low-frequency　energy　harvesting　capability.