The invention relates to an electric generator guard ring detection method based on a grazing excitation inner surface wave. The electric generator guard ring detection method comprises the followingsteps: the probe of a phased array ultrasonic flaw detector is adopted to excite surface wave on the inner wall of a guard ring to be detected through a sound wave grazing incidence way; in addition,the surface wave is circumferentially spread on the inner wall of the guard ring; when the surface wave has defects, reflection wave is generated; the reflection wave is received through an energy converter and is converted into an electric signal to be recorded or displayed in real time; according to a record result or an image displayed in real time, defects on the guard ring to be detected canbe determined. According to the method, the defect is subjected to image record detection, accuracy is high, meanwhile, only multiple points need to be selected on the circumference of the guard ringto carry out axial scanning to realize the 100% scanning of the inner wall of the guard ring during detection, and detection efficiency is greatly improved.