Three Ni/SiO ₂-γ-Al ₂O ₃ catalysts with varying molar ratios of Si to Al were prepared by incipient wetness impregnation. The acid property and crystalline phase of these catalysts were characterized by FT-IR of adsorbed pyridine and XRD, respectively. The activities of the above catalysts were also investigated by the catalytic cracking experiment of mixed plastic. With decreasing of molar ratio of Si to Al, the total acid amounts of catalyst were increased, which induce gas and carbon yield increased and yield decreased. However, excess acid amount caused deep cracking, and then the gasoline fraction yield decreased. The 2 ^# catalyst of all the acid catalysts prepared is the best in this work and exhibits a very high catalytic activity (the gasoline fraction yield exceeds to 40%) on the catalytic cracking process.The gasoline compositions in the products of catalytic cracking were characterized by gas chromatograph. The results show that the acid property of the catalyst can be improved by modulating the molar ratio of Si to Al, and therefore the selectivity of the catalyst can be changed to get more gasoline fraction with high octane number.