Low frequency vibration is a ubiquitous energy existing in our environment, but a large efficient harvesting of which remains challenging. This paper presents a simple piezoelectric spring architecture based on a common binder clip structure. The harvester with pendulum spring allows the energy of the dynamic mass to be converted into electrical energy in the piezoelectric transducer. Due to the basic characteristics of spring pendulums, the proposed harvester can efficiently scavenge not only ultra-low frequency but also multi-directional vibrational energies. Modeling and design are conducted and a normalized expression of the harvester behavior is given. Chirp and human motion excitations are used to evaluate the proposed harvester's performances. Simulation and experimental results are in good agreement. The proposed device could generate a high output power (13.29 mW) at a low operating frequency (2.03 Hz), which shows great application prospects in the power supply of wearable products, ocean buoys, etc.