To improve the security of a rotary piezoelectric generator and broaden the effective frequency band, a rotating piezoelectric generator with tunable frequency was studied and its performance was evaluated through theoretical analysis, simulations, and experiments. Natural frequency offset and stiffness offset models were first established, and the effects of stiffness and the number of rotating magnets on the response performance of a generator were determined through a simulation. The analytical results show that the stiffness of the generator linearly increased with axial tension and linearly decreased with axial compression. In addition, the natural frequency increased and became steady with axial tension but decreased at a fast rate with axial compression. Based on the fact that the number of rotating magnets affects the number of resonant peaks as well as the amplitude ratio of the generator, a prototype rotating piezoelectric generator was fabricated and tested. The test results reveal that the natural frequency of the generator increases and that generated voltage decreases depending on whether the piezoelectric beam is stretched or compressed, with the latter having a greater influence on voltage reduction. In addition, the number of rotating magnets affects the natural frequency of the generator. By changing the number of rotating magnets and conducting pre-stretching and pre-compression, the natural frequency of the generator can be adjusted to within a range of 39.2-112 Hz with a maximum frequency shift of 185.7%.