In this paper, a two-degree-of-freedom parametrically excited piezoelectric energy harvester is proposed by mounting a spring between the piezoelectric beam and the end mass block. Using the extended Hamiltonian principle, the dynamics equation of the harvester is established. The simulation results show that as the piezoelectric beam in straight state, the motions of the piezoelectric beam subsystem and the mass block-spring subsystem affect each other, and the stiffness of the spring and the mass of mass block have an important influence on the performance of the harvester. When the natural frequency of mass block-spring is twice that of piezoelectric beam, the harvester is of an auto-parametric resonance, the excitation threshold of the harvester is the minimum; meanwhile the bandwidth of energy harvesting will be extended.