The invention discloses an addition agent for zinc-copper composite mechanical plating, and belongs to the technical field of hardware surface treatment. The addition agent comprises, by mass percent, 3% to 6% of copper sulfate, 1% to 2% of sodium citrate, 1% to 2% of sodium bentonite, 0.3% to 0.5% of salicylic acid, 6% to 10% of stannous chloride, 10% to 15% of citric acid, 10% to 13% of polyethylene glycol 1000 and the balance water. The addition agent is in a water solution state, and is convenient to use, during adding use, the agent can be directly weighed and used, further expansion is not needed, during use, the adding amount is related with the surface area of a product to be subject to steel plating, components of the addition agent are common raw materials, the raw material source and purchasing are convenient, by the adoption of the addition agent, a zinc-copper composite mechanical coating can be prepared on the surface of the steel product, and dual effects of surface protection and decoration can be obtained.