Abstract: The hydrocracking reaction rules of tetralins and naphthalenes mixture on Ni-Mo catalyst containing HY zeolite were investigated using two series of feeds: feedⅠwith constant tetralin content and various alky-naphthalene contents, feedⅡwith similar total aromatic hydrocarbon content and various tetralins to naphthalenes ratios. The conversion and selectivity of hydrocarbons took part in reactions were calculated from the detailed product analyst. Results showed that for feedⅠthe conversion of naphthalenes decreased with the increase of naphthalene content, the selectivity of tetralins ring opening to form alky-benzene was not affected significantly in this case, yet it was suppressed obviously in feedⅡcase with the increase of methyl-naphthalene content. Due to the occupation of hydrogenation sites and acid sites by the adsorption of polycyclic aromatic hydrocarbons, the hydrogenation and isomerization of tetralins were suppressed, especially for isomerization ring opening reactions.