The invention relates to water-based undercoat composition for vehicles. A component A is prepared from 6.0%-8.0% of an aid, 10.0%-15.0% of pigment, 25.0%-35.0% of filler, 30.0%-35.0% of modified water-based hydroxyl acrylic resin, 3.0%-8.0% of a waterborne polyurethane dispersion, 0.1%-2.0% of an adhesion promoter and 3.0%-8.0% of a cosolvent; a component B is prepared from 65.0%-70.0% of an isocyanate curing agent and 30.0%-35.0% of a cosolvent. The undercoat is a double-component polyurethane system, a film is crosslinked through reactions and has excellent water resistance, flexibility, shock resistance and adhesion, and the undercoat has excellent polishing property, high drying speed and matching property with base coat and topcoat.