An underwater low light imaging optical system is designed to improve imaging effect, resolution and sensitivity of underwater video. Image intensifier assembly is used to increase the sensitivity of underwater monitoring system. Optical lens and illumination light sources with large relative aperture and large field of view (FOV) angle are used at the same time. An underwater low light level monitoring imaging objective with 1/2 relative aperture and 50°underwater FOV angle is designed with 10 optical elements through optimization combination based on inverse telephoto structure. Quality assesments show that the optical system has an excellent imaging quality, modulation transfer funtion (MTF) is better than 0.5 at 65 lp/mm, and all the specifications meet the requirements of underwater low light level monitoring.