The invention provides a photoelectric composite leaky coaxial cable, a leaky cable monitoring system and a detection method. The photoelectric composite leaky coaxial cable comprises an inner conductor, an insulated layer located outside the inner conductor, an outer conductor located outside the insulated layer and a sheath located outside the outer conductor, wherein at least one marking line is arranged on the sheath; and each marking line is internally provided with an optical fiber. At least one optical path is provided for remote optical fiber for system signals and online leaky cable monitoring; due to the leaky photoelectric composite cable with two marking lines, at least two optical paths are provided, requirements on the optical paths by remote system signals and online leaky cable monitoring can be met at the same time. a photoelectric coaxial cable is realized, mounting of the optical cable is attached to mounting of the leaky cable, the strength is firm, and once mounting of the leaky cable is completed, the light paths finishes laying, and the mounting is convenient.