In order to detect corona discharge in all weather situations, a three-band optical system worked at three bands of the solar blind UV, visible and the long infrared was designed for the corona detection. To reduce the system volume and improve the performance merits, a co-lens system was used for both the solar blind UV and visible bands, and an independent lens system containing aspheric surfaces and diffraction surface was adopted for the long infrared band to correct monochromatic and chromatic aberrations. Furthermore,the focusing system was designed with a single lens to ensure an excellent imaging in the temperature from -40 ℃ to 60 ℃. The image quality of the system was evaluated,and it is shown that at the temperature of 20 ℃, the MTF at the spatial frequency of 60 lp/mm is greater than 0.37 in the solar blind UV, and greater than 0.35 in the visible band for the whole fields of view; the MTF at the spatial frequency of 20 lp/mm is greater than 0.6 in the long infrared band for the whole fields of view. These results indicate that the system has excellent imaging characteristics at all three bands, and can be used in the detection of corona discharge in all weather situations.