The invention relates to mildewproof flame-retardant tunnel decoration paint, a preparation method and application thereof. The paint comprises the following components in parts by mass: 10 to 30 parts of water, 1.0 to 2.0 parts of dispersant, 1.0 to 2.0 parts of antimildew agent, 10 to 20 parts of titanium dioxide, 20 40 parts of flame retardant, 20 to 40 parts of acrylic ester polymer, 1.0 to 3.0 parts of nanometer modified coating reinforcing agent, 1.2 to 2.5 parts of film-forming additive, 0.2 to 0.6 parts of defoamer, 1.2 to 2.5 parts of polyurethane thickener, and 0.5 to 3.5 parts of inorganic nano-materials. The preparation method mainly comprises the steps of adding needed components under certain conditions, uniformly mixing or grinding the components, adding the needed component in turn, uniformly mixing all raw materials and then adjusting the mixture to have appropriate viscosity. The paint is applied to the decoration of tunnels, aboveground buildings and underground buildings, and has the characteristics of fire prevention, flame retardancy, mildewproof property, waterproof property, moisture-proof property, high adhesion, non-shedding property, strong resistance tohigh-pressure water impact, long lifespan and the like.