According to the theories of the solid physics and irreversible thermodynamics, the performance characteristics of a novel high-efficient graphene thermionic power device (TPD) are studied. The temperature of the cathode plate and anode plate are determined by solving the energy balance equation of hot and cold sides of the TPD. The effects of the output voltage and the work function of the cathode on the volt-ampere characteristics of the TPD and the temperature of the two electrodes are analyzed to determine the parametric characteristics of the TPD at the maximum power density and efficiency. The power density and efficiency are compromised, and the parametric optimal designs are given. The influence of the temperature of heat source at high temperature on optimization performance is analyzed. The results obtained here can provide theoretical guidance for developing the thermionic energy conversion devices.