In order to separate sodium chloride and sodium bromide from the solid solution Na(Cl,Br) crystallized from bittern, the isothermal solubility data of NaCl–NaBr–CH3OH ternary system at 273 and 323 K was measured by isothermal solution equilibrium method, and the solid phase points were determined according to the measured liquid phase points and wet slag phase points, from which the phase diagrams at two temperatures were obtained. The results showed that the phase diagram characteristics of the ternary system were similar at 273 and 323 K. There was only one invariant point and two univariant curves, and there were three solid-phase crystallization regions: NaCl pure salt crystallization region, the co-crystallization zone of NaCl and Na(Cl,Br) solid solution and Na(Cl,Br) solid solution crystallization region. This was different from the phase diagram of NaCl–NaBr–H2O system, which only had the crystallization region of solid solution. The liquid composition of the invariant point at 273 K was 0.2904wt% NaCl, 14.66wt% NaBr and the liquid composition of the invariant point at 323 K was 0.2529wt% NaCl, 13.45wt% NaBr. The mass fraction of NaBr at the boundary of the solid solution crystallization region at 273 and 323 K was 28.93% and 34.28%, respectively. The solubility of NaBr in anhydrous methanol was much higher than that of NaCl, which indicated that NaBr had a strong salting-out effect on NaCl, and the solubility of these two solutes in methanol at 273 K was higher than that at 323 K, which was opposite to that in aqueous solution. According to the phase diagrams of NaCl–NaBr–CH3OH system at 273 and 323 K and NaCl–NaBr–H2O system at 298 K, the process for the separation of sodium chloride and sodium bromide was designed. Not only the pure salt of NaCl was obtained, but also solid solution with extremely high NaBr content was obtained. And the mass fraction of NaBr in the separated solid solution was 98.06% and 98.15%, respectively.