The invention discloses a method and system for detecting the length of a cable. The method comprises the following steps of: injecting traveling waves into a cable sheath from the head end of the cable in a coupling mode; obtaining the time difference between the traveling waves arriving at a first test point and a second test point on the cable; and determining the length of the cable between the first test point and the second test point according to the time difference and the wave velocity of the traveling waves in the cable. The traveling waves are injected into the cable sheath from thehead end of the cable in a coupling manner, and the cable length between any two test points on the cable can be obtained by detecting the pulse along the line, so that the problem that the cable length is not associated with the actual laying position of the cable is solved, the searching speed of fault distance measurement is increased, and accurate positioning can be achieved without applyinga high voltage to break down a fault point on the cable.