Abstract: Due to the attenuation and scattering effects of light propagation in the water, the details of the optical imaging are severely lost, the contrast is reduced and the color shift is distorted, resulting in the atomization of underwater light image. Therefore, in the poor light conditions, the effective capture range of high-performance underwater cameras is relatively short, and the satisfactory imaging result is hard to obtain. Sonar uses sound wave to detect long distance target due to low attenuation of sound propagation in water. Therefore, when the underwater optical detection device cannot perform accurate optical imaging to capture the long distance target, the information collected by the sonar system can be used to fuse with the optical image information to enhance the image and improve the imaging effect. In this paper, an underwater image enhancement method based on sonar information fusion is proposed. First, the underwater optical image is preprocessed in the two steps, i. e. the dark channel prior model based defog enhancement and the adaptive image enhancement, and then the sonar information is used to locally enhance the underwater image and significantly improve the contrast and recognizability of the target to be detected in the underwater environments.