Abstract: The present invention discloses an aqueous primer composition comprising: (A) an aqueous dispersion of a non-chlorine-based modified polyolefin comprising the dispersion of an unsaturated carboxylic acid- or acid anhydride-modified polyolefin (a), having a melting point of 120° C. or lower and a weight average molecular weight within the range of 50,000 to 150,000, in an aqueous medium; (B) a water-soluble or water-dispersible polyol resin; and (C) a pigment, wherein the solid content weight ratio of component (A) to component (B) is within the range of 20/80 to 85/15, and component (C) is contained within the range of 0.5 to 200 parts by weight based on 100 parts by weight of the total resin solid content in the composition; and a method for coating a plastic molded product using the aqueous primer composition.