Power cable for power transmission has been gradually extended. The system can be still running 1-2 hours when single-phase-to-earth fault occurs, which requires the location of the fault is detected in the case of UPS. A method of power cable fault location online in neutral point ineffectively grounded power system based on HHT is proposed in this paper. Under the condition that the parameters of cable are unknown, the velocity of traveling wave in cable can be measured by the use of switching overvoltage wave which is generated when the closing occurs. When single-phase-to-earth fault happens, HHT is used to find the mutations moments of instantaneous frequency. Then the location of power cable fault is determined by one-terminal fault location method. Finally, the digital simulation results of an example show the effectiveness and practicability of this method.