This paper proposes the design of a novel piezoelectric energy harvester for scavenging mechanical energy excited by human footstep during walking. The device involves a four-stage force amplification using wedge mechanism, leverage mechanism, half-bridge mechanism, and bridge-type force amplifier. It is designed to magnify the input force exerted on a piezoelectric stack so as to enhance the power output. Analytical model of the energy harvester is established via the force analysis based on stiffness matrix method. The main architectural parameters of the device are optimized by adopting multi-objective genetic algorithm (MOGA), which is verified by performing finite element analysis (FEA) simulation study. Moreover, a prototype is fabricated for experimental investigation. Experimental results show that the harvester exhibits a larger force amplification ratio and higher power output than existing works.