Marine plankton imaging technology plays an important role in the study of Marine ecosystem, and has great significance to fishery production and basic research of Marine science. In order to adapt to the observation of Marine plankton, a zoom micro-optical system with large relative aperture made for underwater plankton observation was designed. The imaging environment simulated seawater medium and this system was designed by Zemax software. The system used a 8.8 mm×6.6 mm CCD camera, and the imaging system multiplier were 1, 2, 4, respectively. The numerical aperture remains 0.15 during the process of changing times. The field of view was 8 mm×6 mm~2 mm×1.5 mm. At the maximum field of view, the optical transfer function (MTF) of near-axial field of view and 1 field of view at Nyquist frequency 50 lp/mm was both greater than 0.4, and the distortion of the imaging system was controlled within 3%. The microscopic imaging system structure is simple, and can satisfy the plankton imaging of small size, providing the analysis of the plankton and phytoplankton population research with technical support.