In this paper, we developed an electromagnetic ocean wave energy harvesting device. It consists of a rack, a swing body, transmission gears and electromagnetic power module. The swing body can convert the low-frequency wave energy to mechanical rotation energy, and the transmission gears amplify the low rotating speed of swing body. The electromagnetic power module is used to convert rotating mechanical energy into electricity. On the basis of the flume experiment, this device is expected to produce a maximum output power of 122 mW under wind speed of 20 m/s and its output power density can reach to 0.24 mW/cm³. Therefore, this device can power the small components of the equipment and extend the life cycle. Apart from this, the device is small and can be integrated into the marine monitoring equipment.