Abstract: A piezoelectric energy harvester with two-stepped variable thickness cantilever beam was proposed. The natural frequency of structural vibration was analyzed based on the transfer matrix method and finite element simulation. The influences of the geometric parameters of the beam on the natural frequencies were discussed and compared with the results of the two methods, a good agreement was obtained. Finally, the frequency-response curves of the output voltage were simulated for the energy harvester. Results show that the proposed structure is more effective than the traditional uniform beam for the low-frequency energy harvester. When the natural frequency of the energy harvester, which was designed by controlling the geometric parameters, is near the ambient vibration frequency, the energy harvester obtains high output voltage. This investigation can provide theoretical support for low-frequency energy harvesting that can be used for self-powered low-power electronics.