The invention discloses a method for reducing the content of impurity ions introduced during preparation of oxides. The impurity ions comprise one or more of K<+>, Na<+>, SO4<2-> , X<-> (X=F, Cl, Br and I) and the like, and cannot be completely removed by water washing or in a subsequent roasting stage. The washing solvent in the method is an organic substance which has chemical polarity, and is low in boiling point and miscible with water, and is one or a mixture of methanol, ethanol, propanol, acetone and the like. The filter cake of an oxide precursor is subjected to at least one time or finite repeated times of water washing and then is further washed by the above solvent, so that finally the obtained oxides have substantially reduced impurity content.