An object to be achieved by the present invention is to provide a method capable of forming a multilayer coating film having excellent smoothness, distinctness of image, and chipping resistance, by a 3-coat 1-bake process comprising successively applying an aqueous first colored coating composition, an aqueous second colored coating composition, and a clear coating composition to a substrate, and heat-curing the resulting three layers of the multilayer coating film all at once. The present invention provides a method for forming a multilayer coating film, which employs a 3-coat 1-bake process comprising sequentially applying an aqueous first colored coating composition (X), an aqueous second colored coating composition (Y), and a clear coating composition (Z) on a substrate; and heat-curing the resulting three layers of the multilayer coating film all at once, wherein the aqueous first colored coating composition (X) contains an acrylic resin (A), a curing agent (B), and a urethane resin emulsion (C), and a first colored coating film formed from the aqueous first colored coating composition (X) has a water swelling rate of 100% or less and an organic solvent swelling rate of 300% or less.