According to the performance requirements of hydrocracking catalyst with high selectivity to middle distillate, multi-hydroxyl carboxylic acid (MHCA) and phosphate (AP) were used to achieve combined dealuminum modification of ultra-stable Y zeolites. The modification effect was influenced by the concentration of MHCA and AP, liquid-liquid ratio, reaction temperature and reaction time. In order to find the optimum modification condition, orthogonal experiment was adopted. L(27)(3(13)) orthogonal layout with five factors and three levels was designed and their orthogonal interactions were also included. After investigating the coupling effect of modification systems, the structure and properties of modified samples were characterized by means of BET, FT-IR, NH(3)-TPD, XRD and TG-DTA. The optimum modification condition was obtained from variance analysis. When the modification was operated under 100 degrees C lasting 8 h with MHCA 0.4 mol/L, AP 0.2 mol/L and liquid-liquid ratio 1.5, the modified USY samples could have developed secondary pore 0.184cm(3)/g, 43.7% of the whole pore volume, and appropriate acid distribution as well as good crystallinity. And then 1 L scale-up test was done. The results showed that the secondary pore volume of modified samples could reach 0.145 cm(3)/g, 43.9% of the whole pore volume, and the amount and intensity of medium strong acid as well as the thermal stability of modified USY were favorable. Performance evaluation was carried out on a 200mL fixed-bed single stage hydrogenation unit using Daqing VGO as feedstock. The 140-370 degrees C middle distillate yield was 66.04%, and middle distilate selectivity could reach up to 80.43%. Compared with commercial catalyst, the yield and selectivity were increased by 5.62% and 4.05%, respectively. This result showed that combined modification using multi-hydroxyl carboxylic acid and phosphate could meet the need of productive middle distillate in the industrial unit, which predicted it might be a potentially industrialized method of Y zeolites modification. (C) 2010 Elsevier B.V. All rights reserved.