The invention discloses a hydrocracking catalyst and a preparation method thereof. The catalyst is prepared by a carbonization method and is amorphous silica aluminum subject to hydro-thermal treatment. The amorphous silica aluminum prepared by the method has the characteristics of large specific surface, large pore volume, centralized pore distribution and appropriate acidity; an enough channel is provided for reactants, the reactants and products are convenient to enter / exit, the active ingredients of the carrier are greatly increased, and the distribution uniformity of the acid centre in the pore channel is improved; and moreover, as a larger space and a specific surface are provided for loading metal, the hydrogenated metal has higher dispersibility, the metal utilization rate is improved, and the catalyst provided by the invention has appropriate cracking activity as well as high hydrogenation activity and medium oil selectivity. The catalyst provided by the invention is particularly suitable for finishing the single-stage hydrocracking process once.