The hydrolysis of magnesium chloride hexahydrate and the countermeasures against hydrolysis were investigated. X-ray diffraction, TG-DSC analysis, and chemical analysis were employed to characterize the hydrolysis process, and then the hydrolysis reactions occurred at different temperatures were determined. After that, the combination of inhibition of hydrolysis and conversion of hydrolysate was adopted to cope with the different hydrolysis reactions. The hydrolysis process of MgCl2 center dot 6H(2)O includes the hydrolysis of dehydrated magnesium chloride below 300 degrees C with MgOHCl center dot xH(2)O as hydrolysate and the hydrolysis of anhydrous MgCl2 over 400 degrees C with MgO as hydrolysate. The inhibition of hydrolysis by forming dehydrated ammonium carnallite (below 300 degrees C) and the conversion of hydrolysate (above 400 degrees C) can effectively resolve the hydrolysis problem occurred at different stages. The dimension of the mixture of MgCl2 center dot 6H(2)O and NH4Cl influences the mass transfer and reaction process. When the ratio of height to diameter of the mixture was over 9:1, the magnesia content in anhydrous MgCl2 reached 0.1 wt.%.