The invention belongs to the technical field of electrolytic copper foil machining, particularly relates to an additive for preparing high-performance ultrathin two-side smooth copper foil. Electrolyte per 10000 parts is prepared from the components in parts by weight: 0.1-0.5 part of 2-mercaptobenzimidazole, 0.8-1 part of polyethyleneimine alkyl salts, 0.5-1 part of wetting agent for acid copper,10-30 parts of collagen powders and 0.1-1 part of cerium oxide. According to a lithium-ion battery two-side smooth copper foil prepared by using the additive for preparing the high-performance ultrathin two-side smooth copper foil, degree of curling of the copper foil can be controlled to be below 4 mm, and thus the problem that the degree of curling of the copper foil causes uneven coating during preparing a lithium-ion battery can be improved, and meanwhile, copper foil own internal stress is reduced, fold-resistance times can be up to 9000 times, the folding resistance is greatly improved,and a cohesive force between the copper foil and active materials is improved. The prepared copper soil has good mechanical flexibility, in the condition of bending or twisting repeatedly, electrodestructures are not subjected to damage, and electrochemical performance has no apparent changes.