The optical absorption property of the nanocrystalline La_1–x Sr_x B₆ powder and thermionic emission property of the La_1–x Sr_x B₆ polycrystalline bulk are investigated. As a result, the transmission wavelength of LaB₆ is red-shifted from 591 nm to 658 nm with the increase of Sr doping content. The emission tests indicate that the thermionic emission current density of La_1–x Sr_x B₆ polycrystalline bulk increases from 2.3 A/cm² to 19.36 A/cm² with the increase of La doping content under an applied voltage of 2000 V. The first-principle calculation results reveal that Sr doping LaB₆ leads plasma frequency energy and Fermi level to decrease, resulting in the tunable characteristics of transmission wavelength and enhancement of thermionic emission. Therefore, the Sr-doped LaB₆ as a multifunctional ceramic has a potential application in the field of optical filter or becomes a promising cathode for microwave device.