The invention relates to an ultralow-frequency energy harvester and belongs to the technical field of energy harvesting, solving the problem that the energy harvester in the prior art is small in output energy, low in energy harvesting frequency, and narrow in load application range. The energy harvester comprises a simple-pendulum-like assembly, a piezoelectric composite beam, a fixed magnet, a movable magnet, a connecting shaft and a packaging shell; the rotating end of the simple-pendulum-like assembly is rotationally connected with the connecting shaft, the swinging end of the simple-pendulum-like assembly freely swings in the axial direction perpendicular to the connecting shaft, the fixed magnet is arranged at the swinging end; the fixed end of the piezoelectric composite beam is fixedly connected with the connecting shaft, and the movable magnet is arranged at the suspension end of the piezoelectric composite beam. The ultralow-frequency energy harvester can be used for converting mechanical energy generated in the movement process into electric energy.