Abstract: Based on the analysis for propagation characteristics of the light traveling in water, several essential factors in underwater photography, such as energy attenuation, field loss and water proof, are discussed. The regularity of image quality degradation due to defocus was analyzed when ordinary cameras installed with water proof windows were used under water. To achieve high quality images in deep water exploration, the design concept of the underwater zoom lens is proposed based on the focal power division formula, and an example is given for further explanation of the design consideration. The relative aperture of the zoom lens is F/1.6 in this design. The image quality of the zoom lens is improved, and the maximum FOV vignette is avoided.