A double-sided copper foil substrate, which comprises: a polymer layer; a first copper foil; an adhesive layer formed on the polymer layer such that the polymer layer is sandwiched between the adhesive layer and the first copper layer; and a second copper foil causing the adhesive layer to be sandwiched between the second copper foil and the polymer layer, wherein the polymer layer and the adhesive layer have a total thickness of from 12 to 25 μm. The present invention further provides a flexible printed circuit board structure utilizing the composite double-sided copper foil substrate of the present invention, wherein the second double-sided copper foil substrate has a trench for exposing a portion of the adhesive layer. The double-sided copper foil substrate of the present invention are lower in rebound and satisfying the demand for the greater number of bending and sliding cycles under a lower R angle, and particularly is suitable for thin and flexible electronic products.