The invention mainly relates to a catalyst for preparing ethylbenzene and styrene by alkylating side chains of methylbenzene and methanol. The catalyst mainly aims at solving the problems that when catalysts used in the prior art are applied to a methylbenzene and methanol side chain alkylation reaction, the methanol utilization rate is low, and the catalysts are inactivated easily. According to the catalyst, under the condition of methylbenzene and methanol side chain alkylation, raw materials are in contact with the catalyst, so that the ethylbenzene and the styrene are generated; the catalyst is an X molecular sieve with the mole ratio of silicon in SiO2 to aluminum in Al2O3 ranges from two to three; before the catalyst is used, two or more of potassium ions or rubidium ions or cesium ions are used for conducting ion exchange on the X molecular sieve; and then, a rare earth element, namely lanthanum or cerium, and one or more metal elements of iron, cobalt, chromium, zirconium and bismuth are loaded onto the molecular sieve catalyst. By means of the catalyst, the mentioned problems are well solved; and the catalyst can be applied to industrial production for preparing the ethylbenzene and the styrene through the methylbenzene and methanol side chain alkylation reaction.