The invention provides a waste-drilling-fluid decoloring agent, and a preparation method and application thereof. The preparation method comprises: adding 0.1-0.5 g of a complexing agent into 50 mL-70 mL of water, stirring and dissolving; under the state of continuous stirring, successively adding 7.5-12.5 g of a nonionic monomer, 22.5-37.5 g of a cationic monomer, 0.02-0.10 g of a reducing agent and 0.08-0.40 g of an oxidizing agent, introducing nitrogen at the speed of 1.0 L/min-1.5 L/min for 10 min, heating to 35-55 DEG C, shaking for 1-2 h, stopping shaking and reacting for 2-4 h, so as to obtain a gelatinoid; and performing purifying and drying processing on the gelatinoid, so as to prepare the waste-drilling-fluid decoloring agent. The invention also provides the waste-drilling-fluid decoloring agent which is prepared by employing the above preparation method. The provided waste-drilling-fluid decoloring agent prepared by employing the above preparation method is high in decoloring efficiency, is capable of effectively removing COD in a waste drilling fluid, is less in generated flocs, and is applicable to decoloring processing of polysulfonate waste drilling fluids, waste oil-in-water drilling fluids, waste high-temperature-resistant anti-sloughing drilling fluids and the like.