Recent years have witnessed explosive increase in the number of wearable devices in the market and industry. However, hardly have these devices gained the ability to capture energy from hosts and then get self-charged. In this paper, we design and build a novel wearable electromagnetic energy harvester to scavenge the kinetic energy of human ankle during walking or running. The design is composed of mainly three parts: a spring-mass system, rolling ball pair and the electromagnetic transduction mechanism. The harvester adopts an array of alternating south- and north-pole magnets. This arrangement allows the array exhibits a unique phenomenon, i.e. abrupt magnetic flux density changes within the array. Because of this phenomenon, the harvester displays excellent performance such as relatively high voltage and high power output. We then conducted FEM analysis to validate the hypothetical abrupt flux density changes. A prototype was fabricated for experimental studies. We investigated open-circuit voltage output, current output, and power as well as charging performance into energy storage components. The result shows that harvester possesses excellent performance with the maximum output voltage of 8.64V, peak-peak power of 700mW and the highest volume power density of 24.9mW/cm3. The energy harvester, as a renewable portable power source, can be of great significance for powering smart wearable electronic devices and health care monitoring sensors.