This paper proposes a hula-hoop energy-harvesting system that integrates a hula-hoop motion transformer with an electromagnetic generator. The hula-hoop motion transformer converts the linear motion from environment, machinery, or even human body into rotational one under specifically designed dynamic conditions. Then, the system can generate power through electromagnetism after combining the electromagnetic generator with the rotational motion. The equations prescribing the relation between induced voltage and power for the system are derived according to Faraday's theory. Meanwhile, the physical model of energy-harvesting system is established and the governing dynamic equations are derived as well via the Lagrange's equations. Good agreement of induced voltage and power between theoretical and experimental results is obtained. The maximum power that the system can generate is approximately 5 mW when the frequency and amplitude of the external excitation are 8 Hz and 11.2 N, respectively.