The invention relates to the technical field of metal surface treatment, and discloses a steel normal-temperature quick-acting environment-friendly blackening agent and application thereof. The blackening agent comprises 1%-3% of copper sulfate, 0.5%-4% of nickel sulfate, 0.5%-3% of polyhydroxy aldehyde, 3%-8% of phytic acid, 0.5%-3% of citric acid, 2%-5% of inorganic acid, 0.5%-2% of alkylphenolethoxylates, 1%-3% of ammonium molybdate and the balance water. A blackening method comprises the first step of oil removing, the second step of cleaning, the third step of blackening, the fourth stepof washing neutralization and the fifth step of oil immersing closing. The blackening agent does not contain selenium, and when the blackening agent is applied, rapid blackening treatment can be conducted on the steel surface at the normal temperature, and a blackening film obtained after treatment is flat, uniform and resistant to friction.