The invention relates to a preparing method and application of high-molecular-weight ashless dispersant. The preparing method includes the steps that first, an aliphatic amine and aromatic amine mixture is prepared by the mass ratio of 1:2-5:1; the mixture and polyisobutene maleic anhydride and base oil react to prepare polyisobutene succinimide; polyisobutene succinimide and trimesic acid react to prepare the high-molecular-weight ashless dispersant. The high-molecular-weight ashless dispersant prepared through the process has the advantages of being high in molecular weight, high in soot dispersing performance and capable of being used for preparing diesel engine oil at the CH-4 grade or above.