The invention provides a method for improving the productivity of a device for producing ethylbenzene from dilute ethylene. An existing device for producing ethylbenzene from dilute ethylene is additionally provided with a small liquid phase alkylation reactor A, and an ethylene raw material source is supplemented to determine the running mode of the small liquid phase alkylation reactor A and an existing gas phase alkylation reactor B. When pure ethylene is adopted as a raw material, the parallel connection mode of the reactors A and B is preferably selected, a raw material benzene undergoes heat exchange or is heated by a heat carrier and then enters the reactor A from the lower part, supplemented ethylene enters all catalyst beds of the reactor A in a segmented manner and undergoes a reaction, the effluents of the reactors A and B are mixed, and the obtained mixture enters a coarse separation tower and a subsequent existing separation system; and when an ethylene-rich gas is adopted as the raw material, the serial connection mode of the reactors A and B is preferably selected, the supplemented ethylene reacts with the benzene in the reactor A, the obtained reaction effluent undergoes flash evaporation, the obtained liquid phase component enters a benzene tower and the subsequent separation system, the obtained gas phase component is mixed with an existing dry gas raw material, the obtained gas mixture enters the reactor B and reacts with the benzene, and the effluent of the reactor B enters the coarse separation tower and the subsequent existing separation system. The method can flexibly improve the productivity of the device for producing ethylbenzene from dilute ethylene and improve the purity of the ethylbenzene product.