The invention relates to a pretreatment device, in particular to an on-line pretreatment device for coating copper on the surfaces of stainless steel wires, and belongs to the technical field of electroplating. The pretreatment device comprises a pickling tank(1), a rinsing tank(2) and a sealed container(3), wherein the top of the rinsing tank(2) is enclosed and provided with an air inlet(5), theupper part of the rinsing tank(2) is provided with a plurality of spray headers(7), the lower part of the rinsing tank(2) is divided into a plurality of zones, the liquid level of the first zone is the lowest, and the liquid level of the last zone is the highest; the number of spray headers(7) are equal to that of ladder zones; the upper part of the first zone is provided with an overflow opening(9), while the bottom is provided with a water outlet(10); the upper part of the last zone is provided with a water inlet(8); the sealed container(3) is connected with an outlet of the rinsing tank(2); and the sealed container(3) is provided with an air outlet(6) formed above the electroplating liquid level. By the pretreatment device of the invention, a thick and stable copper-coated layer can beobtained, the bonding force between the coating and the basal body is effectively improved, and the water consumption of the rinsing tank is low.