The present invention discloses a method for detecting corona discharging and a device thereof. The method includes steps as follows: A. ultraviolet imagery instrument image-forming; B. collecting theimage-forming data to a computer through an image collecting card; C. processing through an image processing program: processing data collected by an image collecting module through the image processing program, the image processing includes: intercepting picture in the video, extracting image, processing image layer extractor to the image, spatial filtering processing, then valve quantity, stating an irradiancy area and a total area in the image, then comparing and obtaining the irradiancy area ratio, finally, obtaining the irradiancy area ratio for characterizing corona strength which includes: a ultraviolet imagery module, an image collecting module, an image processing module, a display module, the image collecting module connects with the ultraviolet imagery module and the image processing module, the image processing module connects with the image collecting module and the display module respectively. The device has simple operation, stable testing and reliable data which can provide direct and obvious data for user.