Taking the saline lake bischofite and NH_4Cl that was removed with the ammonia method and continuously followed by filtration as raw materials with a molar ratio of 1∶1 of MgCl_2 to NH_4Cl, ammonium carnallite was synthesized. And then the ammonium carnallite was dehydrated to some extent at 160℃ for 4 h. Ammonium carnallite reacted with ammonia at 240℃ for 150 min and the ammonation ammonium carnallite was produced. Finally, the ammonation ammonium carnallite was calcined at 750℃ into anhydrous magnesium chloride containing only 0.1%(mass fraction) of MgO. On the other hand, dehydrated ammonium carnallite was mixed with the solid ammonium chloride at mass ratio 1∶4 at high temperature and with the differential pressure of NH_3 above 30.5kPa. The dehydrated ammonium carnallite of mixture was dehydrated at 410℃, and then calcined at 700℃ into anhydrous magnesium chloride with only 0.087%(mass fraction) of MgO. X-ray diffraction and electron microscopy analysis results prove that anhydrous magnesium chloride obtained by both methods hasn’t mixed phases, the particle is large and even has good dispersion, which is suitable for preparation of metal magnesium in the electrolysis. Taking the saline lake bischofite and NH_4Cl that was removed with the ammonia method and continuously followed by filtration as raw materials with a molar ratio of 1: 1 of MgCl_2 to NH_4Cl, ammonium carnallite was synthesized. And then the ammonium carnallite was dehydrated to some extent at 160 ° C for 4 h. Ammonium carnallite reacted with ammonia at 240 ° C for 150 min and the ammoniacal ammonium carnallite was produced. Finally, the ammoniacal ammonium carnallite was calcined at 750 ° C into anhydrous magnesium chloride containing only 0.1% (mass fraction) of MgO. On the other hand, dehydrated ammonium carnallite was mixed with the solid ammonium chloride at mass ratio 1: 4 at high temperature and with the differential pressure of NH 3 above 30.5 ° kPa. The dehydrated ammonium carnallite of mixture was dehydrated at 410 ° C, and then calcined at 700 ° C into anhydrous magnesium chloride with only 0.087% (mass fraction) of MgO. X-ray diffraction and electron microscopy ana lysis results prove that anhydrous magnesium chloride obtained by both methods has not mixed phases, the particle is large and even has good dispersion, which is suitable for preparation of metal magnesium in the electrolysis.