The invention belongs to the technical field of a plating copper brightening agent used for circuit board through electroplating, and particularly relates to a copper brightening agent. In order to achieve the technical aims, the copper brightening agent can achieve the characteristics of plating layer surface uniformity, good malleability, high strength of extension and the like. To achieve the above technical aims, according to the technical scheme, the copper brightening agent comprises 50-100 g of polyethylene oxide epoxypropane monobutyl ether, 10-50 g of polyethylene glycol, 10-30 g of acationic polymer, 5-15 g of dithiobis-1-propanesulfonic acid disodium salt and the balance water per litter of the copper brightening agent. The copper brightening agent has the beneficial effects that the plating layer surface is uniform, and the lustrousness is moderate; a plating copper layer has good malleability, and the average elongation is larger than or equal to 25%; the plating copper layer has good strength of extension, and the average strength of extension is larger than or equal to 300 MPa; and the deposition rate is high and ranges from 0.4 micrometer/min to 0.5 micrometer/min,the uniform plating capacity is good, 2A/dm<2>, the uniform plating capacity is larger than or equal to 94%, and the face copper deposition rate is lower than the hole copper deposition rate.