Abstract: A new method to design FCC catalyst for producing lighter olefin and diesel was developed and the USYbased catalyst was synthesized by means of this method. The USY zeolites ultrastablized in the hydrothermal treatment were further modified with tartaric acid in unbuffed system. The crystal structure, surface area and pore distribution,thermal stability and microactivity for catalytic cracking of modified samples were respectively characterized by XRD, N2adsorption,DTA and MAT techniques. The result shows that the Si/Al ratios,crystallinities and thermal stabilities of modified USY samples are effectively improved;simultaneously, the crystal cell size and particle size are obviously decreased. The pore system (micropore and secondary mesopore) is positively developed and the pore distribution is obviously optimized. The optimization of pore and acid sites results in the preferable production of lighter olefins and diesels in FCC process. This means that FCC reaction on modified USY zeolite leads to deep cracking in micropores and partial cracking in mesopores. Optimization of this mo dified USY zeolite might result in a novel FCC catalyst.