A continuous zoom optical system for underwater that has the characteristics of a large field of view, high resolution, and high zoom ratio at the same time is designed to satisfy the needs of deep-sea exploration and achieve higher quality optical imaging in the deep sea. According to the operating deep sea environment, the image quality degradation caused by the extrusion deformation of the optical window caused by deep water pressure was considered, an integrated optomechanical analysis of the optical window was carried out, and the surface shape change results were substituted into the optical system in the form of a Zernike polynomial for optimization. After studying the characteristics of underwater optical aberration and the design method of zoom system, the optical system adopts the mechanical negative group compensation zoom method and image square telecentric design scheme. The system's working distance is 5 m, the F-number of the whole zoom is constant 3.0, and the system can achieve the full field of view of 5.7°-90°, 10 times continuous zoom. The zoom system uses three aspheric surfaces with a total length of 260 mm. At 208 lp/mm, the maximum distortion of each zoom position of the system is <3% and the modulation transfer function values of the entire field of view in the whole zoom area are >0.3. The zoom system features a compact structure, good imaging quality, and a smooth zoom curve, which can accommodate real application demands.