Currently, commercial underwater lenses are mainly those that have small field angles and large optical lengths. Based on the initial structures of six glass lenses and the characteristics of underwater imaging lenses, this study designs a 5-megapixel ultra-short focal underwater wide-angle monitoring lens using an ultra-short focal length scheme. Its full field of view is 95°, the focal length is only 3.1 mm, the total optical length of the system is 32.8 mm, and the rear working distance is 5.3 mm. The lens is composed of a waterproof window and a lens group consisting of four spherical and two aspherical lenses (including a filter and an aperture stop). When the lens operates underwater, the modulation transfer function is greater than 0.3 at a frequency of 250 lp/mm in the entire field of view, and the optical performance of the central field-of-view system is close to the diffraction limit. With an IMX675 sensor, the lens achieves a good imaging effect. An analysis of temperature and tolerance shows that the lens can adapt to changes in water temperature and meet the requirements of practical production and assembly.