In order to realize autonomous target detection,recognition and positioning task of autonomous underwater vehicle(AUV),basing on the side-scan sonar data and considering the characteristics of side-scan sonar images,a lightweight deep learning target recognition model is designed in this paper based on Darknet framework for metal balls. And the characteristics of metal balls are analyzed combining with artificial characteristics. At the same time,an effective image enhancement method is designed for sonar images. Experiments show that the objects recognition method described in this paper not only ensures the accuracy of target recognition,but also provides a high processing rate,which is suitable for the low-power embedded platform.