The invention discloses an S-mode response system in an amplitude comparison direction-finding system, and the S-mode response system comprises a digital orthogonal transformation unit, a digital orthogonal signal preprocessing unit, an A / C / S mode inquiry signal decoding unit, an S-mode response PPM / PCM coding unit, a DA chip data configuration unit, a data processing unit, a response signal azimuth angle control unit, and an omnidirectional / directional antenna control unit. The S-mode response system can refer to the design thought of a hardware circuit of a comprehensive monitoring system on a radio frequency receiving and transmitting channel, cooperates with a dedicated TCAS antenna to achieve a function of directional receiving and transmitting of a radio frequency signal, and greatly solves a problem of asynchronous crosstalk caused by ground secondary radar station inquiry. Moreover, the system increases the detection distance and reduces the power of a transmitter through employing receiving and transmitting gains of the directional antenna. The software logic algorithms of inquiry signal azimuth solving and response signal azimuth angle control are all integrated in an FPGA. The system is short in processing time, is good in timeliness, and is high in digitalization degree.