The present invention relates to a method for flame retardant treatment of a water-soluble flame retardant composition for wood. The flame retardant composition on the surface of wood treated with an aqueous paint permeates the wood, thereby preventing the spread of flame. The present invention allows the preparation of an environment-friendly flame retardant composition having good carbonization areas, after-flame time and after-glow time and having no harmfulness of bleaching, exfoliation or stains on the color of the aqueous paint, and enables flame-retardant treatment only by spray coating or brush coating without using other aids, at room temperature and atmospheric pressure. The present invention is characterized by preparing a water-soluble flame retardant composition for wood by adding 60 g of ammonium phosphate to 300 g of distilled water (base water) followed by 30 minutes of stirring, adding 40 g of urea followed by 30 minutes of stirring, adding 40 g of phosphoric acid followed by one hour of stir
