Aiming at the technical problems of high internal thermal stress and difficult processing in the cerium doped rare earth halide single crystal, an insitu annealing treatment method was presented in this paper. By using this method, the SymbolFC@25.4 mm×25.4 mm Ce∶LaBr3 crystal and SymbolFC@76.2 mm×76.2 mm Ce∶LaBr3 crystals were annealed, and no cracking occurred after cutting, rolling, grinding, polishing and packaging. By testing the energy spectrum of the above two kinds of Ce∶LaBr3, the energy resolution was 2.92% and 3.02% respectively, and the method did not affect the scintillation performance of the crystals. The experimental results show that the proposed insitu annealing treatment method of cerium doped rare earth halide single crystal can effectively eliminate the thermal stress inside the crystal and improve its mechanical properties, which is of great significance for improving the yield of crystal processing.