The reaction conditions of magnesium phosphite prepared by acid-base neutralization were studied.The effects of reaction of magnesium carbonate with phosphorous acid at different ratios on the reaction products were investigated.The products were characterized by XRD,potentiometric titration,redox titration and other methods.The results showed that magnesium phosphite hexahydrate was formed when the amount-of-substance ratio of phosphorous acid to magnesium carbo-nate about 1.0.An acid salt was formed when the amount-of-substance ratio of phosphite to magnesium carbonate about 2.0,that is,it can be formed 0.33 magnesium phosphite hydrate.There was a tendency to form amorphous substances when the amount-of-substance ratio of phosphite to magnesium carbonate was far away from 1.0 and 2.0.The mass fraction of magnesi-um phosphite was 72.66% when the amount-of-substance ratio of phosphite to magnesium carbonate was 1.0 by redox titra-tion,and converted into the mass fraction of six water phosphite magnesium was 92.43%.The mass fraction of phosphorous acid of the product was 84.79% when the amount-of-substance ratio of phosphorous acid to magnesium carbonate was 2.0,and converted into the mass fraction of hydrogen phosphite magnesium was 96.32%.The results of potentiometric titration showed that the curve of magnesium phosphite salt had two jumps,which were consistent with the properties of diacid phosphite.