The invention relates to a plasticization modifying agent for polyvinyl chloride processing and preparation method and use thereof, and it belongs to the field of modifying agent for polyvinyl chloride processing. Said modifying agent is a polymer obtained from monomer composition consisting of 60 to 100 parts by weight of alkyl methacrylate and 0 to 40 parts by weight of alkyl acrylate with total weight of 100 parts, the intrinsic viscosity of the polymer is higher than 7.5. The invention also provides the preparation method of said high viscosity coefficient modifying agent. The invention breaks through the localization and misapprehension in the art, said plasticization modifying agent can effectively improve finish degree of polyvinyl chloride rheological block, enhance plasticization property of polyvinyl chloride. The polymer can be widely used for modification of polyvinyl chloride or composite material containing polyvinyl chloride.