This paper develops a mathematical model of a two degree-of-freedom piezoelectric energy harvester (PEH) in which vibration is driven by disk swing motion. The proposed device converts slow mechanical rotation into piezoelectric vibration using gravity force and magnetic repelling force. The harvester consists of a disk and a piezoelectric cantilevered beam. The disk with an unbalanced mass swings on a rotating object (e.g., wind turbine blade) and two magnets attached to both the beam and the disk can transfer the kinetic energy of the disk to the beam without physical contact. The energy method is used to derive three coupled equations to model the motion of the disk, vibration of the beam, and the piezoelectric voltage output. The effect of harvester orientation on power generation performance is studied as the rotational speed changes, and the simulation results are experimentally verified. Possible application of this energy harvester to a power-sustainable sensor node for large-scale wind turbine blades monitoring is discussed.