To solve the problem of repeated loss of low voltage power cables, this paper designed a project in preventing the cables from stealing. The design of the system is based on the Impedance Characteristics method which is used in power-on state and the Parallel Resonance which is used in power-off state. The paper describes the principle of the method and how to combine the two methods. The simulation and test for it show that the method is correct. The system makes it possible to protect the cable from stealing without affecting the electricity system. In addition, it can calculate the fault location. The system has better accurateness and stableness than the common cable anti-theft system.