Using large pore γ-Al2O3 prepared by acid precipitation, the Ni2P(25%)/γ-Al2O3 catalyst was prepared as the support by the impregnation method. The test results of XRD, BET and mercury porosimetry show that the large pore γ-Al2O3 has preferable surface area and pore structure. After in-situ reduction，the effects of preparation conditions and reaction conditions on dehydrocracking activity of the catalyst were studied in a fixed-bed down-flow reactor with the diesel as the material. The results show that desulfurization degree of 98.2% would be attained under the optimum condition as follows: synthesis temperature of 80 ℃, pH of 8，reaction temperature of 360 ℃，reaction pressure of 4.0 MPa，liquid hourly space velocity of 1.0 h-1 and volume ratio of hydrogen to cyclohexane of 500∶1.