As nonlinear technology allows piezoelectric energy harvesting to obtain a wider vibration frequency and a higher output voltage, this paper proposed a piezoelectric energy harvester based on nonlinear vibration. The oscillation equation of the piezoelectric energy harvester was obtained based on testing Duffing mode and its vibration characteristics were simulated. With the various sinusoidal excitation frequencies and different magnet spacings, the opened output voltage was measured. The results show that when the excitation acceleration is 20 m/s2, the output voltage from the energy harvester has been improved to 208 V from 131 V, its maximum output power is 43.264 mV, and the resonance frequency region can range up to 18 Hz. The Duffing model structure can change the resonant frequency of the nonlinear piezoelectric energy harvester in small scope, and can increase the output voltage.