The differences of circuit design between CINRAD/SB alternating frequencyconversion numerical servo system and those of CINRAD/SA are summarized herein. Based on the signal procedure of servo system and principle of power protection, the fault diagnosis methods of numerical servo system are detailedly described through analyzing the condition monitoring information, file FC information, alarm, key point parameter measurements and key signal transmission paths. Meanwhile, two different typical fault examples are analyzed theoretically. And then some suggestions are given on maintaining the servo system, which are providing useful references to CINRAD techniques.