The invention discloses an absorbent for absorbing and separating polyaromatic hydrocarbon in diesel and a preparation method thereof. The absorbent is characterized by comprising silica gel capable of adjusting an aperture or metal-loaded modified silica gel. The preparation method of the absorbent comprises the following steps: (1) mixing a sodium silicate solution with inorganic acid to form glue, and aging, washing and drying the glue to obtain high-purity silica gel; (2) washing the silica gel by utilizing an appropriate amount of alkaline solution or inorganic acid solution, and directionally adjusting pores of the silica gel; and (3) contacting the silica gel with a metal solution to perform metal modification, and preparing the silica gel or metal modified silica gel absorbent. The absorbent has high adsorption capacity for the polyaromatic hydrocarbon in the diesel, can effectively absorb and separate the polyaromatic hydrocarbon in the diesel and is relatively high in selectivity and reusable.