The invention discloses a method for preparing a blackening coating agent for metal surface treatment. The method comprises the steps that (A) sodium nitrite and copper sulfate are weighed according to the proportion and are dissolved into deionized water; (B) disodium nucleotide, hydroxymethyl cellulose, sodium polyacrylate, poly alkylene glycol, methylparaben and complexing agents are added into an obtained solution, heating is carried out, and stirring is carried out for 20 minutes; (C) the solution in the step (B) is cooled to 25 DEG C, thickening agents and defoaming agents are added into the solution, and stirring is carried out for 15 minutes; (D) intensive mixing and even dispersion are carried out, a 200-mesh bag type filter is adopted to filter the solution, the filtered liquid is the coating agent, and components are weighed by mass. The prepared blackening coating agent can obviously enhance the compactness and firmness of the coating, the problem that the thickness of the coating is uneven is effectively solved, and the blackening coating agent prepared through the method is good in heat stability, innocuous, easily degradable, free of pollution to the environment, and wide in application range.