The invention discloses a sizing agent for derusting and antisepticising the metal surface in the field of a metal material technology and a treatment method thereof. The sizing agent comprises the following compositions in weight percentage: 20 to 30 percent of ferric oxide, 45 to 55 percent of diluted phosphoric acid and 15 to 30 percent of strengthening material, wherein the mass concentrationof the diluted phosphoric acid is between 40 and 50 percent. The method comprises the following steps: the metal surface needing derusting and antisepticising treatment is subjected to heating treatment; the heating temperature is controlled between 35 and 100 DEG C; the sizing agent for derusting and antisepticising the metal surface is brushed on the metal surface subjected to heating treatment; the depth of a coating layer is between 1 and 2 mm; and the maintenance time for the coating layer is between 30 minutes and 250 hours. Under the heating condition, iron, the ferric oxide and the phosphoric acid have chemical reaction to form a crystal with a compact structure and form a whole with metal, thereby realizing the consistent deformation between the coating layer and a substrate. Theraw materials adopted by the method have no toxicity, no pollution, and belong to environment-friendly materials. The method is simple and easy to operate.