Abstract: Effects of polymerization degree of polyglycols on their adsorption on Hg and inhibition of Cu²⁺ discharge were studied. It follows that increasing polymerization degree with widen the potential region of adsorption and enhance the inhibiting effect upon Cu²⁺ discharge, but a little further effect is made when the polymerization degree is more than 100; as a result, the polyglycol with polymerization degree about 120 is suggested to be used as the additive in acidic copper-plating. In this paper the models for the adsorption of polyglycol molecules and the mechanism of inhibition of Cu²⁺ discharge are discussed,by which the regularity on the effects of the polymerization degree can be satisfactorily explained.