The invention provides a method of applying a flue gas desulfurization ash modified material to a wall whitewashing material.Firstly, flue gas desulfurization ash is modified, so that calcium oxide in the flue gas desulfurization ash is converted into calcium hydroxide, calcium sulfite is converted into calcium sulfate, and accordingly modified desulfurization ash is obtained; the modified desulfurization ash serves as a raw material, alum and other auxiliary materials are used, and the whitewashing material used for a wall is obtained through blending processing.The weight percentages of the modified desulfurization ash, alum powder, heavy calcium carbonate powder and a water-retaining agent are 80-95%, 2-8%, 0-15% and 0-5% respectively.The modified desulfurization ash and the alum powder are mixed, aluminium potassium sulfate in alum can be easily hydrolyzed into aluminum hydroxide which belongs to inorganic gel materials, and calcium hydroxide and calcium sulfate are inorganic gel materials.The modified desulfurization ash is the main component of the wall material, and the resource regeneration function is achieved.