The invention relates to a method for preparing styrene with high selectivity through methylbenzene and synthesis gas. The method comprises the following steps of leading raw gas containing the methylbenzene and the synthesis gas into a reactor, enabling the raw gas to be in contact with a dual-function catalyst after hydrogen reduction, and performing a side chain alkylation reaction to prepare the styrene, wherein the dual-function catalyst comprises an alkaline molecular sieve and a metallic oxide. According to the method disclosed by the invention, the synthesis gas is used for replacing methanol for performing the side chain alkylation reaction of the methylbenzene, so that not only can the cost of raw materials be reduced, a dehydrogenation step in a conventional styrene preparation process is omitted, the energy consumption is reduced, but also a hydrogen transfer reaction of the styrene and the methanol in the side chain alkylation reaction of the methylbenzene and the methanol is avoided, the styrene/ethylbenzene ratio of products is increased, and the method has the characteristics that the methylbenzene conversion rate is high and the selectivity of the styrene in the products is high.