Polycyclic aromatic hydrocarbons (PAHs) have been paid more attention as liquid organic hydrogen carriers (LOHCs) because of their high hydrogen storage, easy transportation, low price, and other advantages. Dehydrogenation is the key point of the PAH hydrogen storage. However, the dehydrogenation reaction rate of perhydro-PAHs is slow, and their pathway is still not clear. To clarify the PAH dehydrogenation pathway, three kinds of perhydro-PAHs containing six-membered rings (methylcyclohexane, perhydro-diphenylmethane, and perhydro-dibenzyltoluene) are selected, and their dehydrogenation processes over the Pt/Al2O3 catalyst are carried out by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT). It was found that the dehydrogenation in the six-membered ring started in the para position of the -CH3 group, and then, the six-membered ring was transformed into a benzene ring gradually. Between the six-membered rings, dehydrogenation started from the side ring, which has fewer groups.