The invention relates to an echo detection and self-excitation elimination method for an electromagnetic wave common-frequency amplifying repeater system. By using the method, a relationship among grains of an amplifier, a phase when returned electromagnetic wave signals reach a receiving antenna and the output power of the amplifier in the electromagnetic wave common-frequency amplifying repeater system is discovered. According to the relationship, the magnitude of echo signals can be detected effectively, the output power of the amplifier is improved, the self-excitation of the amplifier resulting from the echo signals is avoided, the requirements on isolations of a forward antenna and a backward antenna are reduced, the system grains are improved, and the coverage area is increased. The method is suitable for all electromagnetic wave common-frequency amplifying repeater systems, such as communication repeaters, data television repeaters and the like.