An experiment is designed to validate the visible imaging system′ s performance through analyzing the effects of resolution, contrast and the illumination. The effective targets at a distance of about 4.5 km are tested instead of 20 km. Under conditions of 11 km visibility, the equivalent detection range is figured out by means of contrast and illumination. If the target′ s illumination is not less than 300 lux, the system can identify the target with 0.253 contrast, but it is not sure when the contrast is between 0.2 and 0.253. With special image processing, the result is significantly improved. Experimental results show that the system′ s performance can meet the special requirements.