The invention discloses a preparation method of an acrylic acid resin retanning agent. At present, acrylic acid resin retanning agents which can simultaneously overcome the defects of pastel shade and plastic feeling are all imported products, and the leather making cost is greatly increased. The preparation method is characterized by comprising the steps that a mixed monomer and a benzoyl peroxide solution are synchronously dropwise added to a mixed solution of ethylene glycol monomethyl ether and water at a constant speed for 2 h to 3 h, and heat preservation is conducted for 1 h to 2 h after dropwise adding is completed, wherein the temperature of the mixed solution ranges from 75 DEG C to 80 DEG C; the temperature in a polymeric kettle is cooled to 30 DEG C to 40 DEG C, ammonia water of which the mass percent ranges from 20%-30% is used for regulating the pH of the system to range from 6.0 to 7.0, and the acrylic acid resin retanning agent is obtained. The prepared acrylic acid resin retanning agent can obviously improve the pastel shade effect in the dyeing process of crust leather, and the plastic feeling of leather is greatly reduced.