A shoe-mounted piezoelectric energy harvester was designed to collecting energy from human walking. This energy harvester comprises of four piezoelectric cantilever beams magnetically coupled with a spring-mass system. The spring-quality system could sense the acceleration excitation along a radial bone shaft and generate electricity through the magnetic coupling driving a piezoelectric element. The expression of acceleration signals was obtained by fitting the experimental data. And then, a simulation model was established to simulate the power generation performance of the energy harvester. Finally, a prototype was also fabricated and its performance was verified by an experimental test. The results indicate that the four beams could be triggered several times by the spring-mass system within one step when the energy harvester is excited by the acceleration along the tibial axis, and several piezoelectric beam voltage outputs could be obtained. Furthermore, the power-generating capacity of the piezoelectric beam excited by the coupled accelerations is always better than that excited by the single direction acceleration. When walking speed ranges from 2 km/h to 8 km/h, the peak voltage output from each beam is as large as 10 V. The piezoelectric energy harvester collects the energy from human walking in low frequency and the superposition of voltage outputs generated by accelerations comes from two orthogonal directions, so that the power-generating capacity is improved greatly.