It is known that the human body contains rich chemical energy, part of which is converted to mechanical energy of up to 200W when in motion. Therefore, it is ideal to harvest human body kinetic energy to power mobile electronic devices. This paper presents an energy harvester to unintentionally extract human kinetic energy and directly convert it to electricity, which can serve as a power source for portable electronics. The proposed harvester mainly consists of an eccentric rotor made of permanent magnet, electric steel and coils as stator. The eccentric rotor, as a simple pendulum, acts as the kinetic energy harvester, which can absorb the motion from the human body during walking. With the permanent magnets on the rotor, the moving rotor can produce a changing magnetic field, where electric coils can induce electricity. A torsion spring is also added onto the rotor such that the harvester works even when placed on a horizontal plane where the gravitational acceleration fails. The electromagnetic and kinematical model is built to analyze the performance. Numerical analysis and system optimization are also conducted. Simulation shows that the harvester with 40mm diameter and 50g weight, worn on the wrist, can produce dozens of milliwatts of electricity during normal walking.