A flexible circuit board includes a first base layer, a circuit layer, a second base layer and a bonding layer. The circuit layer is coupled to a side of the first base layer and includes a linear signal line, two grounding lines located at two opposite sides of the linear signal line, and two hollow areas located between the linear signal line and the grounding lines. The bonding layer couples the second base layer to the circuit layer and defines a first opening corresponding to the linear signal line and communicating with the hollow areas of the circuit layer. The first opening of the bonding layer and the hollow areas of the circuit layer cooperatively define an air medium layer surrounding the linear signal line. A method for manufacturing the flexible circuit board is also provided.