The invention relates to a high-selectivity method for synthesizing styrene through methanol and toluene side chain alkylation. In the method for synthesizing styrene through toluene and methanol alkylation by use of a molecular sieve based catalyst containing metal aid modification, in the presence of a carbon dioxide medium, the mixture of toluene and methanol at a molar ratio of 2.0-8.0 is used as a raw material, the reaction temperature is 350-540 DEG C, the toluene reaction airspeed is 0.1-4.0/h, the pressure of the reaction medium is 0.1-6.0MPa, and the molar ratio of carbon dioxide to hydrocarbon is 0.5-10.0. According to the invention, by taking a large amount of excessive greenhouse gas carbon dioxide as a reaction medium, the styrene is prepared with a high yield, and the problems of many side reactions and poor selectivity in the existing styrene synthesis technology through toluene and methanol alkylation are solved; and moreover, the reaction stability can be improved so that the catalyst stability and styrene selectivity in the styrene synthesis through toluene and methanol alkylation meet the requirements of industrial application.