The invention provides a vehicle tail gas reductant having high reduction efficiency, good stability, and few by-product. The vehicle tail gas reductant comprises 10-55 weight parts of high-purity carbonyl diamine, 35-90 weight parts of ultrapure water, and 0.5-12 weight parts of stabilizing agent. The high-purity carbonyl diamine is obtained by multi-recrystallization and micro-nano active carbon adsorption, thus the impurities which hinderthe reduction of the tail gas are greatly reduced. The stabilizing agent is combined with metal ions by coordination bond or covalent bond, thus the disperse stability is strong, the complexation capacity is high, the complexation stability constant is large, and the reductant can be used with lasting effect. According to the invention, the ingredients of the tail gas reductant use environmental protection and recycling as the basic selection principle and have good biodegradability, thus the reductant can rapidly, efficiently and stably convert NOx and other harmful gases into pollution-free N2 and H2O, the SCR system poisoning caused by reduction residues is prevented, and the requirements of the tail gas reduction conditions of a diesel car SCR system can be satisfied.