The technical features of the AC variable frequency CINRAD/SA Digital Servo System are summarized, and the main channel signal process, monitoring signal process, and frequency converter signal process are described.According to the monitoring information,alarm information,and key points parameter, from the three aspects of position loop,velocity loop, and acceleration loop, a detailed discussion is made on the fault diagnosis methods of AC variable frequency CINRAD/SA digital AC servo system, as well as uncontrolled antenna, abnormal antenna running, jump code or corner code, and inconsistent antenna location. Two typical fault cases, as examples, are discussed and summarized in terms of theoretical analysis and processing steps, and some suggestions for the maintenance of AC variable frequency digital servo systems and CINRAD technology support are given for reference.