The kaolinite and montmorillonite were activated and modified by hydrochloric acid. Then a pillared montmorillonite was prepared by acided montmorillonite and hydroxyl aluminum.The structure of mineral sample was characterized by X-ray diffraction. By using Hammett indicator method and butylamine-cyclohexane solution the authors measured the surface acidity of clay with H₀ value in the range of +4.8~-5.6, and obtained the distribution curves of acid strength by fitting. The authors studied the catalytic performance of clay solid acid by pyrolysis of cumene converted into a model reaction. The result shows that cumene conversion has been improved through being catalyzed by acid-actived clay solid acid, but the catalytic stability is reduced. The modification of montmorillonite by hydroxyaluminum pillars not only greatly increases the conversion of cumene but also improves the catalytic stability.