Abstract: The main purpose was to introduce the acoustic emission location testing for partial discharge of in-service 500 kV extra high voltage transformers. The principle and method of acoustic emission location testing were discussed. The partial discharge of 500 kV extra high voltage transformer was tested by acoustic emission location testing technology and the position of partial discharge was found. The testing data was analyzed and it was consistent with result of electric partial discharge testing and dissolving gas analyzing. So the acoustic emission location testing technology can be used effectively to monitoring and testing on line to the partial discharge of huge in-service electric transformer.