Flexible electronic devices often comprise conductive Cu patterns and polymeric substrates. In this article, copper conductive patterns are readily fabricated on various soft polymeric substrates at room temperature by reduction-assisted sintering with ascorbic acid (AA). Conductive lines on polyethylene terephthalate exhibit good flexibility, adhesion, electrical conductivity, and robust durability under substantial deformation. According to XRD and XPS characterization, pure Cu is obtained after reduction by AA. The sintering of Cu is demonstrated via the mechanical strength of Cu patterns. The optimal concentration of AA is about 0.5 M, which corresponds to the lowest resistivity of around 7.74 × 10-4 Ω cm. The sintering of Cu particles at room temperature can be explained by the removal of oxygen associated with reduction by AA, which drives the rearrangement of nearby Cu atoms. Good adhesion may be attributed to the structure of AA, which is like that of l-3,4-dihydroxyphenylalanine. This method may be applied to spray printing technology or paper electronics.