The present disclosure provides a process for effectively separating and removing by-products (light gas, C5-C6 hydrocarbon oil fraction, polyethylbenzene) contained in a substantial amount in addition to ethylbenzene as a target compound in an alkylation reaction product of a fluidized catalytic cracking off-gas to prepare ethylbenzene having high purity, and process economical efficiency may be secured by utilizing the polyethylbenzene as an absorbent or gasoline fraction, without requiring a transalkylation reaction for further producing ethylbenzene from polyethylbenzene.