For the purpose of energy cascade utilization, a new coupling system consisting of a graphene-based thermionic energy converter and a two-stage thermoelectric generator is proposed, in which the waste heat of graphene-based thermionic energy converter is recovered by the two-stage thermoelectric generator for generating extra power. Based on thermionic emission, semiconductor physics, and non-equilibrium thermodynamics, mathematical equations for power, efficiency, and exergy efficiency of the coupling system are presented, and the general performance features of the hybrid system are revealed. Numerical analysis shows that the maximum efficiency, maximum power density, and maximum exergy efficiency of the coupling system are 54.99%, 7.93Wcm−2, and 68.75%, respectively. Moreover, the maximum efficiency and maximum power density of the coupling system are enhanced by 28.39% and 28.52%, respectively, and the maximum exergy efficiency is improved by 28.39% compared to the stand-alone graphene-based thermionic energy converter. Subsequently, comprehensive parametric analyses are carried out to derive the effects of the heat source temperature, Fermi level of graphene, cathode work function, semiconductor element area-length ratio, and the Thomson effect on the performance of the coupled system. The obtained results may be useful for the design and running of this kind of energy cascade utilization system.