Smart watches and wristbands are helpful in health monitoring, sport training, living assistance, etc. However, due to critical size limitation and high-power requirement, battery endurance becomes a big issue that the devices need to be frequently recharged, which even causes “battery anxiety” among the users. Although capturing the kinetic energy from arm swinging renders a promising solution, it’s a challenging task to produce high power in a limited size. To achieve this, we invented an embedded and highly efficient generator with coaxial topology to integrate motion capture unit, magnetic frequency-up converter and power generation unit in a thin and compact device, which allows it to be embedded in the smart watches and wristbands. Despite of integrating several parts in a highly compact device, the mechanical structure is concise and easy for fabrication. At the same time, to increase the energy efficiency, the mechanical friction is reduced by using magnetic transmission and compact skew-ball-like bearing. The testing results showed that our embedded generator can achieve maximum power of 1.74 mW and normalized power density of 820.38 μW/cm3·Hz2, which are more than 4 and 10 times that of the counterparts.