The invention discloses a technique for producing styrene by alkylating toluene and methanol side chains. The method comprises the steps of mixing and heating methanol vapor and air to obtain a mixture, enabling the mixture to enter a reactor producing formaldehyde by using methanol, and generating formaldehyde under the action of a catalyst; condensing a product, enabling the condensed product to enter a rectifying tower so as to remove water generated in the reaction, pressurizing a product at the tower top, enabling the pressurized product to enter a high-pressure separator, discharging noncondensable gas containing hydrogen from the top of the high-pressure separator, mixing and heating and a product at the bottom and toluene, enabling the mixture to enter a side chain alkylation reactor, enabling a gaseous product to return to therectifying tower after a product obtained in the reaction is subjected to separation by a gas-liquid separator, enabling a liquid product to enter a toluene tower, an ethylbenzene tower and a styrene tower sequentially, enabling toluene to return to the side chain alkylation reactor, and discharging ethylbenzene, styrene and C9<+>heavy aromatics. The technique is of a two-step method for toluene and methanol side chain alkylation, not only can the generation degree of a side reaction between styrene and hydrogen be effectively inhibited, but also the hydrothermal deactivation of the catalyst for side chain alkylation can be alleviated.