This article presents the design, fabrication, and testing of a novel piezoelectric energy harvester. Its unique feature is that a single piezoelectric stack is used for harvesting the energy from two-directional excitation in vertical directions. To accept the input forces in two directions, a flexure-based two-stage force amplifier is integrated with a conical transmission mechanism and a sliding guiding mechanism. An analytical model is established to predict the force amplification ratio of the proposed energy harvester. In order to maximize the output performance of the energy harvester, the dominant parameters of the mechanism are determined, which are optimally designed by resorting to a genetic algorithm. A prototype energy harvester is fabricated for experimental testing. Experimental results indicate that the developed harvester exhibits the advantages of high safety factor, large force amplification ratio, and compact size. It can provide the maximum average output power density of over 286.70 kW · m ^-3 from 10-N force excitation in two directions.