The invention provides a preparation method of a diquaternary ammonium salt. Dimethyl carbonate and a diammonium salt are placed in a reaction kettle, and the diquaternary ammonium salt is prepared under airtight conditions at the reaction temperature of 60-280 DEG C for the reaction time of 0.5-24 h. A substituent of nitrogen in cation [CH3NR1R2(CH2)nN(CH3)R3R4]<2+> of the diquaternary ammonium salt is one or more than one of alkyl, alkenyl, alkynyl and aryl selected from R1, R2, R3 and R4, or is an organic group containing hydrocarbon and one or more than one element of oxygen, nitrogen, phosphorus, sulfur, fluorine, chlorine and bromine; and an anion of the diquaternary ammonium salt is one of sulfate ion, hydrosulfate ion, nitrate ion, phosphate ion, hydrophosphate ion, dihydrophosphate ion, fluorine, chlorine, bromine, iodine, sulfur, carbonate ion, oxalate ion, acetate ion and carboxylate ion. The invention has the advantages of simple process, short production period, high yield and low cost; the diquaternary ammonium salt containing designated anions can be further synthesized; and the product has wide application.