Rare earth borides (REB_x) have been the subject of many theoretical and experimental studies because of their excellent electron emission and magnetic properties. However, it is still challenging to obtain relatively pure rare earth borides with low cost by simple synthetic methods. Herein, an economical method for synthesizing RE borides was proposed, on the basis of reaction between rare earth oxides (RE_xO_y), boron carbide (B₄C) and aluminum (Al). 15 rare earth elements were investigated to identify their stable phases in the system of RE_xO_y-B₄C-Al, respectively. After high-temperature reaction at 1773 K and following alkaline leaching at 363 K, single phase YB₄, LaB₆, CeB₆, PrB₆, NdB₆, SmB₆, EuB₆, GdB₄, TbB₄, DyB₄, HoB₄, ErB₄, TmB₄, YbB₆, LuB₄ were successfully synthesized. Meanwhile, the harmless and resourceful treatment of the leaching solution was done to prepare nano γ-Al₂O₃ and NaCl. Thus, the zero discharge of the whole process was achieved.