Using fish behavior to reflect changes of water quality and aim at water quality warning is currently a popular topic of environmental monitoring. The fish tracking technology with occlusion is the basis for the analysis of fish school behavior, and only tracking each individual fish accurately in the shoal can get the exact quantitative analysis of their feature. In this paper, we design the data acquisition platform by using a plane mirror and a monocular camera, and present a fish tracking with occlusion method by using the three-dimensional information. According to the principle of plane mirror imaging and the camera imaging, we find the relationship between the target in mirror-view and the one in the direct-view. Based on the relationship and underwater imaging process, we calculate the 3D coordinates-for both real fish and imaginary fish under circumstance of occlusion. Finally, we use the 3D coordinate to achieve the target occlusion tracking for fish school. The experimental results show that this approach can obtain precise tracking of three fish and get the 3D coordinate accurately even they are in occlusion.