This paper analyzes the propagation characteristic of modal current in cross bounding power cable, and designs an online fault location algorithm for HV power cable by using current mode 4 as fault location signal. The algorithm is independent of the propagation velocity, which can solve the problem of complicated reflection and refraction of traveling wave on cross-bounded power cable and the problem of inner mode and outer mode wave crossing into each other. By extracting the frequency band of high energy in the transient traveling wave to reconstruct the transient wave, the bandwidths is significantly reduced, thus the influence of wave dispersion is decreased. EMTP simulation and Matlab calculation results show that the fault location algorithm is not only feasible but also has higher accuracy.