The La0.6Eu0.4B6 nanopowders were prepared by high-energy ball milling, and then the ball-milled nanopowder were used to prepare the high-density La0.6Eu0.4B6 polycrystalline bulks by spark plasma sintering (SPS). The influence of sintering temperature and pressure on the sample density and mechanical properties were investigated. The results show that this method reduces the sintering temperature more effectively than the conventional hot-pressing method. The sample density, Vickers hardness and flexural strength are 4.71 g/cm3, 23.37 GPa, and 295.14 MPa, respectively, which ??are also higher than those of conventional hot-pressing methods. The thermionic emission results show that when the cathode temperature is 1873 K, the maximum emission current density of La0.6Eu0.4B6 is 33.74 A/cm2. Under the same sintering condition, the Vickers hardness, flexural strength and thermionic emission current density of ball-milled nanopowder are increased by 28%, 58% and 32% compared with coarse-powder-sintered sample. These results indicate that refining the powder grain size to nano level is beneficial for reducing the sintering temperature and promoting the thermionic emission and mechanical properties.