In order to meet the requirements of deep-sea detection, an underwater optical zoom system with large relative aperture is designed. Characteristics and design principles of underwater zoom system are analyzed. The optical system is a mechanically compensated zoom system; its image size is 8.8 mm×6.6 mm with zoom ratio of 2×. Its F-number is kept constant at 1.4 during zooming process. In its short focal length mode, its field-of-view (FOV) is 66°, while its FOV in long focal length mode is 36° and its effective focal length ranges from 5.9 mm to 11.8 mm. The zoom system employs one high-order aspheric surface and its overall length is 245 mm. The water-air interface of the zoom system is the concentric lens which is able to withstand high static pressure. For short, middle and long focal length positions, the modulation transfer function (MTF) of the system is always larger than 0.6 for in-axis field and 1.0 field at the spatial frequency of 42 lp/mm. Moreover, the maximum distortion for all zoom positions in the whole FOV is less than 5%. With compact design and high image quality, the zoom system can meet the requirements of practical application.