Abstract: Surge arrester is a common equipment in substation, which plays an important role in releasing lightning and system operating overvoltage. Metal-oxide surge arresters without gaps are most widely used. This paper introduces the structure and characteristics of Metal-oxide surge arrester. With the help of the abnormal case of arrester in a substation, the abnormal fault of metal-oxide arrester without gaps is detected by means of live detection, infrared temperature measurement, state detection and diagnostic test. The correctness of various monitoring results and diagnostic test conclusions are verified. The research on the abnormal phenomenon and diagnosis method of metal-oxide arrester without gaps is of great significance to improve the maintenance level of substation operation and maintenance.