A composite cable assembly includes flat cables, a cable unit and fastening units. Each flat cable includes conductor assemblies and a shielding layer covering the conductor assemblies. The cable unit includes transmission lines. The transmission lines are arranged horizontally and in contact with the shielding layer of the flat cable closest to the transmission lines. The cable unit contacts the flat cable closest to the cable unit, the cable unit and each flat cable are together bent to form bent portions and extension sections connected to the bent portions. Each two fastening units are arranged spaced apart at two sides of a corresponding bent portion. Each extension section has the same length when the cable unit and each flat cable are moved in a movement direction to extend or collapse.