The invention relates to the fields of additives of heavy fuel oil and lubricant and especially relates to an ultrahigh-base-number magnesium sulphonate additive and a preparation method thereof. The ultrahigh-base-number magnesium sulphonate additive is mainly prepared from an organic solvent, a basic oil, methanol, vinyl succinic anhydride, magnesium oxide, heavy alkyl benzene sulfonic acid, water, glacial acetic acid and fuel oil. Carbon dioxide is fed during a preparation process. A total base number (TBN) of the ultrahigh-alkali-value magnesium sulphonate additive can reach more than 600 mg KOH/g. The content of magnesium can reach 14.0 wt%. The additive is 40-200 cSt in viscosity at 100 DEG C. The additive is stable in structure, is low in viscosity, is good in oil solubility and is convenient to use. A preparation method of the additive includes following steps: performing a neutralizing reaction between the magnesium oxide and the heavy alkyl benzene sulfonic acid with cooperative effect of the methanol, the glacial acetic acid and the vinyl succinic anhydride in the organic solvent, and then performing a carbonation reaction to obtain an ultrahigh-base-number magnesium sulphonate colloid and performing filtration and distillation to finally obtain the ultrahigh-base-number magnesium sulphonate additive.