Anhydrous magnesium chloride (MgCl(2)), the dehydration product from bischofite (MgCl(2)center dot 6H(2)O) and as industrial raw material for preparation of electrolytic magnesium,. is now the most advanced and perfect technological process. For long, the detailed dehydration process was not known due to its dehydration complexity and lack of appropriate experimental conditions. In this paper, quantum chemistry method based on density functional theory (DFT) was used to study the whole dehydration processes. The molecular geometries Of MgCl(2) center dot 6H(2)O, MgCl(2) center dot 4H(2)O, MgCl(2) center dot 2H(2)O, MgCl(2) center dot H(2)O and MgCl(2) were all optimized at level of B3LYP/6-31G*, the optimized geometrical parameters and correspondent energies corrected by the second order Moller-Plesset perturbation theory (MP2) were thus obtained. Results show that the energy variations corresponding to the whole dehydration steps from MgCl(2) center dot 6H(2)O via intermediates MgCl(2) center dot 4H(2)O, MgCl(2) center dot 2H(2)O and MgCl(2) center dot H(2)O, to anhydrous product MgCl(2) are 35.55, 41.30, 28.55, 31.08kcal/mol, respectively. For steps of 2H(2)O removal, the energy variation from MgCl(2) center dot 2H(2)O to MgC12 is 59.63kcal/mol, bigger than the steps from MgCl(2) center dot 6H(2)O to MgCl(2) center dot 4H(2)O (35.55kcal/mol) and from MgCl(2) center dot 4H(2)O to MgCl(2) center dot 2H(2)O (41.30kcal/mol), which means the last two water molecules are the most difficult to be removed. All these results are significant for mechanism study of bischofite dehydration and are helpful for industrial production of anhydrous magnesium chloride.