Abstract: As thermionic emission cathode material, rare earth hexaboride (REB₆) has been used in various electron emitting devices. As technologies have developed, the requirements for the electron emission and mechanical properties of REB₆ single crystal cathode materials have increased. Doping with rare earth elements can improve the emission properties of the material. The mechanical properties of REB₆ single crystal can be effectively improved by adding high elastic modulus transition metal boride (TMB₂) into REB₆ to form the REB₆ matrix-TMB₂ fiber directionally solidified eutectic composite. Therefore, both doped REB₆ single crystal and REB₆-TMB₂ directionally solidified eutectic composite, which are prepared by floating zone melting, are large-size cathode materials with great application prospects. In this paper, the crystal structure of REB₆ was presented and its influence on the properties was discussed. Then, the thermionic emission properties of binary REB₆ single crystal were summarized, including the effects of crystal orientation and rare earth elements on the thermionic emission properties. The research progress of doped and composite of REB₆ were reviewed, the properties of the materials were summarized, and the factors affecting the properties were discussed.