Extracting characteristic signal from a continuous signal can effectively reduce the difficulty of analyzing the running states of a single-variable nonlinear system.Whether the extracted characteristic signal can accurately reflect the running states of the system is very important.In this paper,a method called automatic sampling method (ASM) for extracting characteristic signals is investigated.The complete definition is described,the effectiveness is proved theoretically,and the general formulas of the extracted characteristic signals are derived for the first time.Furthermore,typical Chua’s circuit is used to accomplish a lot of experimental research on the aspect of frequency domain.The experimental results show that ASM is feasible and practical,and can automatically generate a characteristic signal with the change of the original signal.