Alumina-aluminum phosphate (AAP) was used as a noble metal support, and Pt/AAP was used as a hydrogenation catalyst to reduce the aromatic content in diesel fuels. Naphthalene dissolved in n-hexadecane was used as a model compound to simulate the aromatics in diesel fuels for the hydrogenation activity test. The experimental results indicated that the Pt/alumina-aluminum phosphate catalyst had a better hydrogenation activity and lower cis-Decalin selectivity than Pt/gamma-Al2O3 catalyst due to the higher acidity of the AAP support than that of the gamma-Al2O3 support. The catalysts Pt/AAP, at 6 < Al/P < 10, have the highest hydrogenation activity and the lowest cis-Decalin selectivity of all Pt/AAPx catalysts. By controlling the Al/P ratio, variable pore size and surface area of AAP supports have been obtained. The high surface area and large pore size of Pt/AAP catalyst can be used for aromatic hydrogenation with good activity and an effective diffusion coefficient. When phosphorus was added to Pt/gamma-Al2O3 catalyst, it improved the hydrogenation activity but reduced the cis-Decalin selectivity for the naphthalene hydrogenation reaction system. However, the improvement of activity was not as good as that for Pt/AAP catalyst and too high a phosphorus content (Al/P < 20) of Pt/gamma-Al2O3 was poisonous to the hydrogenation activity and cis-Decalin selectivity.