The invention discloses a structure for improving water/oxygen barrier performance of a flexible substrate and a preparation method of the structure. The flexible substrate is formed by a graphene-based water/oxygen barrier film and a polymer substrate, wherein the flexible substrate is located below an electrode/N-type semiconductor layer/active layer/P-type semiconductor layer or an electrode/P-type semiconductor layer/active layer/N-type semiconductor layer to form a complete device. The preparation method is transferring a graphene film or a graphene composite film to the polymer substrate; any one of a bubbling method, a substrate corrosion method and a heat release method is adopted by the process; and the transfer process is roll-to-roll transferring or small-size manual transferring. By hydrophobic and waterproof characteristics of double layers and graphene, the water/oxygen barrier effect of the flexible polymer substrate is improved; the structure can be universally applied to photoelectric devices with various structures; and the photoelectric efficiency of the device is further improved.