The invention relates to the technical field of ATC responders, and discloses an automatic detecting system of an S mode responder. The automatic detecting system of the S mode responder comprises a control module, a signal processing module and a receiving and sending module, wherein the control module is used for sending inquiry data to the signal processing module, receiving responding data returned from the signal processing module, comparing and analyzing the responding data and originally-stored simulated responding data, and displaying the analyzed results; the signal processing module is used for analyzing and encoding the received inquiry data, then sending the inquiry data to the receiving and sending module, decoding the responding data returned from the receiving and sending module, generating a message format of the inquiry data and then sending the inquiry data to a control module; the receiving and sending module is used for achieving mutual transmission of signals between the receiving and sending module and the responder, and achieving frequency conversion, receiving and sending of the signals. According to the automatic detecting system of the S mode responder, accuracy detecting can be conducted on the specific field of the responding information of the S mode responder, manual detecting errors can be reduced, complex testing steps can be simplified, the testing period can be shortened, and working efficiency can be improved.