A water-based coating composition and method for forming a multi-layer coating film by which it is possible to form a coating film 5 having excellent smoothness, image clarity, and chipping resistance in a 3C1B format. The composition is characterized by containing 7 to 60 mass % of an acrylic resin emulsion (a), 10 to 60 mass % of a film-forming resin (b), and 10 10 to 50 mass % of a curing agent (c), on the basis of the total mass of the solid resin content in the composition; the acrylic resin emulsion (a) having a core/shell structure where the core section contains, as a copolymer component, 0.1 to 10 mass % of a polymerizable unsaturated 15 monomer having two or more polymerizable unsaturated groups in one molecule, on the basis of the total mass of monomer of the core section, and the hydroxyl value of the core section is 50 to 200 mgKOH/g, and the hydroxyl value of the shell section is 50 to 200 mgKOH/g.