The invention discloses a water-based nano flame-retardant polyurethane floating coat paint and a preparation method thereof. The invention aims to provide a water-based nano flame-retardant polyurethane floating coat paint for rail transit vehicles, which has the advantages of favorable physical and mechanical properties, excellent adhesive force, high drying speed, favorable filling property and flame retardancy. The preparation method comprises the following steps: proportionally mixing a water-based hydroxyacrylic acid dispersion, a functional pigment and filler, a dispersing and wetting agent, a defoaming agent, a leveling agent, a thickener, a pH stabilizer, a halogen-free environment-friendly flame retardant, water and aliphatic hydrocarbon isocyanate; and carrying out dispersion, grinding, color matching, packaging and other steps to prepare the 2K water-based nano flame-retardant polyurethane floating coat paint.