The invention discloses an acrylic acid-modified epoxy polyester resin and the preparation process thereof. The resin comprises 100 pbw of a copolymer (A) formed from an unsaturated fatty acid-modified polyester and a bisphenol A type epoxy resin, and 20-40 pbw of poly(meth)acrylic acid (ester) (B). The preparation process comprises the following steps: (1) alcoholyzing a drying oil with a polyol; (2) adding successively, to the product from step (1), a dibasic anhydride, a fatty acid containing a conjugated double bond and an epoxy resin, and esterifying to obtain an epoxy polyester; and (3) copolymerizing the epoxy polyester from step (2) and an acrylic monomer. This invention also provides a water-based emulsion containing said resin, and a water-based paint exhibiting excellent antirust property and having low production cost.