The purpose of this research was to study the influence of the aluminum content of Ni/Al–SBA-15 catalysts on catalytic performance in the production of carbon nanotubes (CNTs) and hydrogen (H₂) during waste plastic gasification. The effects of the preparation method (i.e., impregnation (IM) or the polyol process (P)) and Ni loading on CNT production were also discussed. Al–SBA-15 supports were synthesized by a two-step “pH-adjusting” hydrothermal method with various Si/Al ratios. The surface acidity of the Ni/Al–SBA-15 catalysts could be altered without destroying the mesoporous hexagonal structure by varying the Si/Al ratio. The catalytic performance indicated that the catalyst loaded with 10 wt% Ni on Al–SBA-15 with an Si/Al molar ratio of 10 prepared by the polyol process (10Ni/Al–SBA-15(10)–P) produced higher quality CNTs with uniform diameter in 74.1% yield. The mesoporous hexagonal structure and the more strongly acidic sites of 10Ni/Al–SBA-15(10)–P enhanced the CNT quality and promoted the dehydrogenation and aromatization of the waste plastic. Moreover, the Ni/Al–SBA-15(10) catalysts prepared via the polyol process with higher Ni loadings possessed better Ni dispersion and more active sites for high CNT yield compared to impregnated catalysts. Increasing the aluminum content of the Ni/Al–SBA-15 catalyst also enhanced the dry reforming of methane and hydrocarbons to generate more H₂ in the waste plastic gasification process.