The invention discloses a dyeing wastewater treating agent and a preparing method thereof. The dyeing wastewater treating agent is prepared from aluminum hydroxide gel, sodium metaaluminate, lime, grain starch, sodium silicate, sodium lignin sulfonate, activated carbon, a powder auxiliary, urea and water. The aluminum hydroxide gel, the sodium metaaluminate, the sodium silicate, the sodium lignin sulfonate, the activated carbon and the powder auxiliary are sequentially added to a reaction kettle, the lime is added into water to prepare lime water, the lime water is slowly added into the reaction kettle while being stirred, heating is conducted at a constant temperature, and stirring is even; then, cooling is conducted, and the grain starch and the urea are added to be evenly stirred at the constant temperature. According to the dyeing wastewater treating agent and the preparing method thereof, organic pollutants and heavy metal in wastewater can be effectively removed, the settling time of aggregates can be shortened, the cost of wastewater treatment is reduced, and the efficiency of wastewater treating is improved.