We present a method based on image pixel value to evaluate the characteristics of the corona discharge of polymer insulators. To capture the images of the solar-blind ultraviolet (SBUV) band, an ultraviolet (UV) imaging detector based on an image intensifier is designed. First, through the experiments’ results with the UV light source, the proposed method based on the image pixel value is proved to detect solar-blind signals more effectively than the photon counting method. Then, by conducting experiments with the insulator at a fixed observation distance, the corona characteristics of the insulator can be described through analyzing the images captured by the detector. Finally, the experiments at different observation distances are conducted, and the image pixel value ratios of the recorded images with certain observation distance are computed. The results reveal that the developing stages of different corona discharges can be uniquely described. Based on the method, the characteristics of a corona discharge in the SBUV band can be described accurately and objectively.