Abstract: Carrying out the alkylation of benzene with propene over zeolites (especially ZSM-5, Y, mordenite, and β) instead of AlCl3 or phosphoric acid as catalysts can be an environmentally friendly and noncorrosive process without the release of acidic catalyst components. Performing the reaction under supercritical conditions is effective to alleviate the catalyst deactivation caused by the carbonaceous deposition. However, it is still necessary to modify the zeolites to improve the catalyst life time and to decrease the contents of by-products that may have undesired effects on the quality of subsequent products. Therefore, in this work, the properties of Hβ zeolite modified with citric acid and its catalytic behaviours on the alkylation of benzene with propene under supercritical conditions were investigated. The results indicate that the catalytic behaviour of Hβ zeolite like catalyst life, n-propylbenzene content, di-isopropylbenzene selectivity and the isomers distribution is related to the acid strength and density of acidic sites of the catalyst, which can be modified with the treatment of citric acid. High density of acidic sites with high acid strength is favorable for the transalkylation, but aggravates the catalyst deactivation, which is mainly caused by the blocking of pore structure due to coke deposition. The concentration of citric acid has a significant influence on the properties of treated Hβ catalysts. Low concentration of citric acid may enhance the acid strength and density, while high concentration may result in the decrease of acid density and the distortion of pore structure. Hβ treated with the citric acid solution of 0.5mol/L owns high density of weak and moderate acidic sites; by carrying the alkylation over it, the catalyst life prolonged about 30% and n-propylbenzene content decreased about 90% in comparison with that over the untreated Hβ catalyst.