The invention discloses an online composite insulator X ray scanning robot. The robot comprises an automatic walking mechanism arranged on a shell (8), an X ray detection mechanism, and a computer on the ground; the automatic walking mechanism moves up and down along a guide rail (7), which is arranged on two sides of a vertical string of composite insulators (1); a X ray detection mechanism scans the composite insulators (1) at intervals and sends to scanning information to the computer on the ground so as to obtain the X ray image of continuously scanned composite insulator (1) after reconstruction; and the X ray image can be used to analyze the defects of the composite insulators (1). The robot has the advantages of simple structure and convenient operation, can detect the online composite insulators (1), which are in operation, under a charged state, and is capable of accurately finding various defects of the composite insulators (1) during the operation process in time.