A hydrogenation and lightening method for monocyclic aromatic hydrocarbons is disclosed. The method includes bringing a catalytic diesel oil raw material, a hydrogen-containing gas and a monocyclic aromatic hydrocarbon hydrogenation and lightening catalyst into contact under conditions for monocyclic aromatic hydrocarbon hydrogenation and lightening. The catalyst comprises a carrier and a metal element loaded on the carrier. The carrier is prepared by bringing a NaY molecular sieve and an organic acid into contact in a water-containing solvent to obtain a modified NaY molecular sieve, wherein the shape of channel openings of the modified NaY molecular sieve is horn-shaped. The method reduces contents of sulfur and nitrogen in diesel oil, reduces the yield of a diesel fraction and increases the yield of a gasoline fraction.