The invention discloses a composition based on diatomite modification and used for purifying tail gas of a grain drying machine, and a preparation method and application thereof. The preparation method comprises the following steps: 1) calcining bentonite in the presence of protection gas so as to prepare bentonite powder; 2) placing active carbon powder, wheat bran powder, fly ash and bentonite powder in an acid solution and carrying out activation so as to obtain an activated mixture; 3) subjecting MDI, TDI, polyether polyol, diatomite, tetrabutyl titanate and the activated mixture to a contact reaction in the presence of supersonic waves so as to prepare foamed resin; and 4) crushing the foamed resin so as to obtain foamed resin powder and mixing an anionic surfactant, water and the foamed resin powder so as to obtain the composition based on diatomite modification and used for purifying tail gas of the grain drying machine. The composition prepared by using the method has excellent capability in purifying tail gas of the grain drying machine; and the preparation method is simple to operate and uses easily available raw materials.