A flexible flat cable for high-frequency signal transmission comprises an inner flat cable and a pair of shielding composite layers, and is divided into a basic section, two connecting sections and two sealing sections. The two connecting sections extend from two sides of the basic section, and the two sealing sections respectively extend from the two connecting sections; wherein the inner flat cable is located in the basic section and has a plurality of conductive wires, an insulating adhesive layer, a pair of insulating spacers, and a pair of metal shields. The insulating adhesive layer coats the conductive wires, the pair of insulating spacers are respectively located at the two sides of the conductive wires and are fixed by the laminating of the insulating adhesive layer, and the pairof metal shields are located at the outer side of the pair of insulating spacers respectively. The width of each shielding composite layer is larger than that of the inner flat cable; each shielding composite layer comprises an outer shielding layer positioned on the outer side and a conductive adhesive layer positioned on the inner side; the conductive adhesive layer has one part attached to theinner flat cable and the other part attached to each other.