Abstract: The low-level-light (LLL) night vision machine can effectively observe the target under the poor illumination conditions at night, and the target contrast observed at night is an important indicator of the influence factors of the whole operating distance. In order to meet the urgent need of target contrast data for night observation theoretical analysis and test of LLL night vision machine. Based on the use characteristics of the 3^rd generation LLL image intensifier with good matching with the natural light radiation spectrum under starlight conditions, an imaging device based on the 3^rd generation LLL ICCD is built. From the energy transfer chain of LLL imaging system and the visual characteristic equation of optoelectronic imaging system, the relationship between the ambient illumination, with the target-background reflectance ratio and the contrast is deduced respectively. The contrast test was carried out in dark room and natural night conditions. The test results show that when the illuminance is in the linear correlation illuminance interval E_s, E_m of the third generation LLL image intensifier, the contrast has nothing to do with the illuminance; for the same target and background, the target-background reflectance is closer to 1, the lower the contrast is.