Railway signal cable is long and under ground, which leads to difficulties in cable fault detection. The second correlation method is improved. The improved computation model based on FFT is designed, and its real time computation ability is analyzed. Based on the improved second correlation method,a signal cable sequence time domain reflectometry／spread sequence time domain reflectometry (STDR/SSTDR) detection model established. Moreover, the open and short circuit fault detection performance of the model is tested at various SNRs. The result shows that SSTDR based on the improved second correlation can be used to detect the signal sectioncable fault.