The present invention relates to alkyl aromatic compound production method, (a) an alkylating agent containing an input stream (input stream) and alkene (alkene) containing the aromatic hydrocarbon, the preliminary with a first 1 SiO ₂ / Al ₂ O ₃ ratio the method comprising: introducing a preliminary alkylation system including an alkylation catalyst (preliminary alkylation catalyst), the preliminary alkylation catalysts are molecular sieves (molecular sieve) which, with the introducing step, (b) alkylation conditions for the production of said alkyl aromatic compound in a step of operating the preliminary alkylation system, the conditions are, the aromatic hydrocarbon is at least in part, on the condition that the liquid phase, and the step, (c) wherein the alkyl aromatic compound and unreacted aromatic hydrocarbon from the preliminary alkylation system recovering a first output stream comprising and, (d) first alkylating containing a first alkylation catalyst having at least a portion with an alkylating agent of the first output stream, the first 2 SiO ₂ / Al ₂ O ₃ ratio the method comprising introducing into the system, wherein the first alkylation catalyst is a molecular sieve, the preliminary alkylation catalyst and wherein the first alkylation catalyst, the preliminary alkylation catalyst has a lower SiO ₂ / Al ₂ O ₃ ratio than the first alkylation catalyst the gajyeoseo, any alkylation catalyst is replaced (replacement), regeneration (regeneration) or reactivation (reactivation) a to remove that frequency (frequency) is the alkylation catalyst alone in comparison reduction that point on, another difference is that the introduction step and, (e) a step of operating a first alkylation system under alkylation conditions for the production of said alkyl aromatic compound, the above condition, the aromatic hydrocarbon is at least in part, on the condition that the liquid phase, the step, (f ) a step of recovering a second output stream comprising the alkylaromatic compound from the first alkylation system.