Abstract: A small fixed fluidized bed device (ACE Model C) was used to study the cracking reaction path and coke formation mechanism of model compound decaphthalene over Y molecular sieve catalyst at the reaction temperature of 460−540 ℃. The results show that in the initial stage of decalin cracking, H ⁺ attacking C−H bond and C−C bond with tertiary carbon atoms on decalin to form non-classical five-coordinated tertiary positive carbon ions is the most important initiation reaction. Decalin cracking products are mainly propylene, propane, isobutane, isopentane, methylcyclopentane, toluene, dimethyl benzene, etc. The yields of the products on the catalyst from large to small are: non-aromatic hydrocarbons, monocyclic aromatic hydrocarbons, bicyclic aromatic hydrocarbons. The catalytic coke formation mechanism of decalin is carbocation mechanism. With the increase of reaction temperature and molecular sieve acid content, the bilayer hydrogen transfer and dehydrogenation condensation ability are enhanced, and the coke yield and conversion are also increased.