In order to monitor the working status of marine ecological monitoring instruments, a three-gear zoom optical system with large relative aperture is designed. This system is designed with Zemax software and its total length is 200 mm. The system uses a 6.4 mm×4.8 mm CCD camera, and the three-gear zoom focal lengths are 8, 14, and 28 mm, respectively. The relative aperture remains at 1/1.4 during zoom process. The maximum field of view is 52° with short focus. At maximum field angle, when the Nyquist frequency is 42 lp/mm, the modulation transfer function (MTF) of the system at 0 field of view is 0.8, the average MTF is 0.7 at 0.707 field of view, and the average MTF value at 1 field of view reaches 0.6. In 10 μm range, the diffraction encircled energy is 90%, and distortion control is in a reasonable range. The system has high zoom ratio, simple structure and large relative aperture. It is suitable for monitoring the marine ecological monitoring instruments and can timely feedback their working status information. Therefore it greatly reduces the maintenance cost of marine ecological monitoring equipment.