The invention provides a low-freezing-point nitric oxide reducing agent and a preparation method thereof. The low-freezing-point nitric oxide reducing agent consists of, by mass, 20%-32.5% of urea, 5%-10% of dimethyl sulfoxide, 40%-55% of pure water and one or more of 10%-20% of acid amide, amides and low molecular alcohols. The preparation method comprises the following steps of preparing raw materials, performing full mixing at the temperature ranging from 10 DEG C to 30 DEG C till all raw materials are completely dissolved into a water solution and mixed evenly, using a filter element with the filter accuracy ranging from 0.5 mum to 5 mum and filtering the raw material water solution under the pressure of 0.2-1 MPa to obtain a finished product. The anti-freezing agent raw materials have the advantages of being safe and easy to obtain, good in anti-freezing effect, non-toxic, harmless and the like. The freezing point of the reducing agent is greatly reduced, the problems of low-temperature crystallization and icing are solved, the anti-freezing performance of the nitric oxide reducing agent can be remarkably improved, and the freezing point can be lower than subzero 25 DEG C. A test shows that after engine exhaust gas is filled into an SCR aftertreatment system of the low-freezing-point nitric oxide reducing agent to be treated, the exhaust gas emission can be up to the national IV standard.