Abstract: A method for extracting a characteristic signal from a power frequency signal includes steps of: S1, detecting a voltage half-wave zero-crossing area in a single voltage cycle in the power frequency signal by a signal; superimposing a voltage modulation signal on the voltage half-wave zero-crossing area, and superimposing an instantaneous pulse on a current cycle in the power frequency signal corresponding to the voltage half-wave zero-crossing area; generating the characteristic signal with current cycles and voltage cycles; S2, demodulating whether there is the voltage modulation signal in the power frequency signal, if so, executing step S3; and S3, demodulating the characteristic signal to obtain the current modulation signal and the voltage modulation signal respectively; and determining whether data corresponding to the current modulation signal and date corresponding to the voltage modulation signal are predetermined data, wherein if so, the characteristic signal exists.