The invention discloses a preparation method of a treating agent for repairing slightly-polluted water. The method sequentially comprises the following steps of adding 20-30g of commercial bentonite powder and 0.1-0.3g of hexadecyl trimethyl ammonium bromide to 40-50mL of water, and stirring for 1-2 hours; adding 0.01-0.05g of nitrilotrimethylene triphosphonic acid, adding 3-5g of calcium chloride, carrying out stirring, precipitating and separating, and removing a supernate; pouring 30-50mL of 20-30% hydrogen peroxide solution and 2-3g of ammonium chloride, putting the whole reaction in a water bath at 4-5 DEG C, reacting for 4-5 hours, precipitating and removing the supernate; and adding 10-20mL of 2-4mol/L ferric chloride solution, stirring for 2-5 hours, precipitating and separating a solid sediment from a liquid, washing the solid by using deionized water for 3-5 times and drying at 105 DEG C to prepare the material, namely the composite material for treating the slightly-polluted water. The material has an adsorption characteristic, large specific surface area and very high adsorbability, and an excellent treatment effect can be obtained through concentration and degradation.