In this paper, two new cable fault detection and location on-line methods — Sequence Time Domain Reflectometry method (STDR) and Spread Spectrum Time Domain Reflectometry method (SSTDR) were introduced and analyzed. An improved frequency correlation algorithm for real-valued sequences was introduced to cable fault detection and location online; it greatly reduced the time and the storage consumption of the detection system. The simulation models of these two methods were established; the feasibility of these two methods and the algorithm was validated by simulation and then compared these two methods, Simulation and comparison results show that SSTDR is a universal cable fault detection and location on-line method.