The increased industrial emissions have worsened the water environment and resulted in the endangerment of marine life. Traditional water body monitoring technology was unable to meet the real-time, fast and accurate application requirements. In order to implement more effective monitoring of the water body environment, various research units have begun to use remote sensing technology to obtain water body information. Aiming at the technical requirements of water monitoring, an off-axis three-reflection optical system is designed with a focal length of 84.5mm, a diameter of 20mm, and a field of view of 36°×6°. The reflective system can meet the requirements for wide spectrum coverage of space remote sensing. The primary and third mirrors of the system adopt sixth aspheric surfaces, and secondary mirror adopts quadric surfaces and the Offner compensator, which takes the primary mirror as an reference, is designed to generate aspheric waves to perform zero compensation test on the aspheric mirror. After tolerance analysis, it has good machinability and the modulation transfer function (MTF) values of each field of view at the Nyquist frequency (71lp/mm) are all over 0.6, which is close to the diffraction limit, and shows good imaging quality. This optical system is expected to have certain reference significance for monitoring water pollution.