The invention discloses a method for extracting a characteristic signal by an active signal method single-phase grounding fault indicator. A fault indicator in a short-circuited loop performs analog-digital conversion through current signal collected by CT (captive test), a built-in CPU (central processing unit) module of the fault indicator performs the algorithm operation and extracts the characteristic signal; the existence of filled signal is judged through analyzing frequency characteristic. The method has the beneficial effects of realizing easier detection of signal filled by the signal source through extracting linear currents overlapped with characteristic current for two times, helping the single-phase grounding fault indicator to judge the single-phase grounding fault, and greatly reducing the false judgment and missed judgment of the fault indicator to the single-phase grounding fault. The method is simple in principle, high in positioning accuracy, low in cost, and easy to popularize.