The development of a high-power and long-life thermionic ceramic cathode poses significant challenges, namely the need for high electron emission capacity at low temperature and excellent machinability. In this paper, the first-principles method was used to obtain the (La0.5Ba0.5)B6 with the lowest work function, and (La0.5Ba0.5)B6–ZrB2 composite with high relative density of 98.08 % was prepared by spark plasma sintering (SPS) at T = 1800 °C, under which the densification mechanism transformed from the particle rearrangement into the grain boundary diffusion, and the corresponding apparent activation energies were 282.75 kJ/mol and 466.54 kJ/mol, respectively. The maximum current density of 12.93 A/cm2 at T = 1500 °C of the composite was higher than that of 11.71A/cm2 at T = 1600 °C of LaB6, which can be attributed to the high Fermi level and weak electronic constraints of (La0.5Ba0.5)B6. The practical work function distribution (PWFD) of 0.36 eV indicated the composite had excellent emission uniformity.