The invention discloses a process for preparing anhydrous bromide LnBr3. In the bromide, cationic element Ln belongs to lanthanum elements, particularly lanthanum La, cerium Ce, praseodymium Pr, neodymium Nd, samarium Sm, europium Eu, gadolinium Gd, terbium Tb, dysprosium Dy, holmium Ho, erbium Er, thulium Tm, ytterbium Yb and lutetium Lu, and the technology belongs to the field of chemical reagent preparation. In the preparation process, a hydrate LnBr3.mH2O and excessive thionyl bromide SOBr2 are used as initiative raw materials to undergo chemical reaction under the protection of high-purity nitrogen at the reacting temperature of between 35 and 40 DEG C in a water bath for 10 to 24 hours, so that the high-purity anhydrous bromide LnBr3 is generated, and the generation of a side product bromine oxide can be effectively inhibited, wherein other products in the chemical reaction, such as SO2 and HBr, are sufficiently absorbed by NaOH alkaline solution, and excessive product SOBr2 can be reclaimed by distilling. The process can prepare the high-purity anhydrous bromide, and can be widely applied in the fields of crystal growth, related lanthanum metal production by a fused salt electrolysis method, related rare-earth material preparation and the like.