This paper presents the design, fabrication, and testing of a dual-axial underfloor piezoelectric energy harvester based on piezoelectric stack. Usually, an underfloor energy harvester can only scavenge the energy from vertical force excitation. In addition to vertical pressure, the floor also suffers from lateral friction force during human walking. One uniqueness of the designed underfloor energy harvester lies in that it can collect energy from both vertical and horizontal excitation forces. To amplify the input force to get a larger output power, a flexure-based two-stage force amplifier is introduced and optimally designed. In order to transmit the two-directional (2-D) input force into the single direction that can be received by the force amplifier, the conical transmission mechanisms and slider mechanisms are introduced. Position limiters are adopted to protect the proposed mechanism. A prototype has been fabricated and tested. Experimental results indicate that the fabricated dual-axial piezoelectric energy harvester can provide an average output power up to 1.25 mW with a load resistance of 12 kΩ from 5 Hz force excitation of 100 N in two directions. With a load resistance of 26 kΩ, the harvested energy per walking footstep is derived as 7.00 mJ for the underfloor energy harvester.