A scheme to mount an elastic magnifier to piezoelectric energy harvester under parametric excitation is proposed in this paper. Using the extended Hamiltonian principle, the dynamics equation of the system is established, and simulations are carried out to investigate the influence of the elastic magnifier and stiffness parameters on the characteristics of energy harvester. The results show that when the piezoelectric beam does not buckle and the natural frequency ratio of the piezoelectric beam to the elastic amplifier meets the resonance condition of 1∶2, the excitation threshold of the parametric excitation energy harvester can be reduced and the frequency bandwidth of the energy harvester can be widened. The stiffness coefficient is named as resonance stiffness coeffieient, ksr. It is indicated that when the stiffness coefficient of the elastic magnifier is less than ksr, the mean power output, the peak of root-mean-square voltage and the frequency bandwidth increase with the increasing of the stiffness parameter. When the stiffness coefficient of the elastic amplitude amplifier is greater than the resonance stiffness coefficient, with the increase of the stiffness coefficient, the frequency bandwidth of the energy harvester increases, the peak value of root mean square voltage decreases slightly, and the average output power tends to be the same when the excitation amplitude is large.