The invention discloses an additive for improving catalytic activity of an alkylation reaction. In particular, a trace of additive prepared from amine salt of alkyl phosphate, pinene, alkyl benzene sulfonic acid and heavy aromatic solvent oil is added to a concentrated sulfuric acid catalyst to catalyze a liquid phase alkylation reaction of iso-butane and alkene, production of by-products and high-carbon alkane is inhibited while yields of high-octane-value oils such as trimethylpentane (including 2,2,4-trimethylpentane, 2,3,3-trimethylpentane and 2,3,4-trimethylpentane) and the like are increased, and the quality of alkylated oil can be improved; when 50-500 ppm of the additive exists in a concentrated sulfuric acid catalyst system, the solubility of iso-butane in a sulfuric acid phase can be effectively increased, the alkylation process can be accelerated, and the loss of the concentrated sulfuric acid catalyst can be reduced.