This paper presents the design, fabrication, and testing of a novel bistable piezoelectric energy harvester dedicated to vibration energy harvesting in the direction of gravity. Its unique feature is that the bistability is realized by combining the elastic potential energy of a bridge-type compliant mechanism and the gravitational potential energy of a proof mass together, which simplifies the preloading mechanism design. As a result, its potential energy traps exhibit less inconsistency under the excitation in gravity direction, which leads to easier skipping between the two energy wells. To evaluate the performance of the proposed harvester, the analytical model is established. Parametric study is conducted to investigate the influence of the structural parameters on the harvester's performance. Static structural analysis is performed to predict the actual performance of the proposed harvester, which is verified by conducting simulation studies. A prototype device was fabricated for experimental testing. In addition, a figure of merit has been introduced for performance comparison versus existing designs. Experimental results indicate that the proposed device is more suitable for scavenging the energy in gravity direction.