Fault line selection and fault location in distribution system when single phase to ground fault occurs are two important aspects. Injection current signal theory is proved to be effective to solve these problems. But present fault location systems based on injection signal are mostly manual one. When fault occurs, an operator is necessary to hold a portable injection signal detector along the fault line to determine fault site, which has a high cost about material resources and manpower. Moreover, a fairly long time is needed and during this course another fault may occur to result in phase to phase fault. An automatic fault site location system based on injection signal is proposed. In this system injection signal detectors are fixed along distribution lines, when fault occurs, a signal with special frequency will be injected into power system, certain detectors will detect the appearance of this signal and then transmit detection results to the sever by GSM-SMS. Sever in substation can determine the fault site automatically according to detection results of detectors and network topology. This fault site location system have been put into application and achieved satisfied operation effects.