In order to harvest the acoustic energy effectively in the environment, an acoustic energy harvester composed of a Helmholtz resonator and the piezoelectric circular diaphragm is proposed in this paper. The incident sound wave is amplified by the Helmholtz resonator. The amplified sound wave acts on the circular piezoelectric vibrator to generate vibration and converts the sound energy into electrical energy through the piezoelectric effect of the piezoelectric plate. The equivalent lumped parameter theoretical model of piezoelectric transducer constituted by circular piezoelectric vibrator is established. The acousticelectric conversion principle of the acoustic energy harvester is analyzed and the influence of the radius ratio of the radius of piezoelectric plate to the radius of substrate on the acoustic energy harvester is studied. The simulation results show that the power generation effect is best when the radius ratio of the radius of piezoelectric plate to the radius of metal substrate (inner cavity diameter) is 0.8. The maximum output power is 12 μW at the incident sound pressure of 1 Pa(94 dB), the acoustic wave frequency of 1 188 Hz and the load resistance of 3.7 kΩ.