Abstract: The effect of the number of crystal seeds on the physical and chemical properties and catalytic performance of the mordenite (MOR) zeolite was investigated using the transalkylation of toluene and trimethylbenzene as the probe reaction. The results show that the addition of seed crystals in the synthesis process will significantly affect the acidity, specific surface area and pore volume of the catalyst, thus affect the activity and stability of the catalyst. When the addition amount of crystal seeds is 8%, the MOR has the most B acid content, the maximum specific surface area and the pore volume, and the activity and stability of the catalyst are also the best. In addition, the mechanism of the transalkylation reaction of toluene and trimethylbenzene has been studied in detail. The results show that the transalkylation reaction in the MOR molecular sieve channels is carried out by the mechanism of bimolecular intermediates, and the intermediates, confirming the rationality of the mechanism of the bimolecular intermediates and deducing the possible reaction route.