This letter studies the problem of target localization in aerial manipulation tasks. When an aerial robot flies close to a target to manipulate, the target would be occluded by the onboard robotic manipulator occasionally or for a long period of time. It is, however, necessary to continuously localize the target even when it is occluded. This is a unique problem in aerial manipulation and has not been addressed in other tasks such as ground manipulation. To solve this problem, we propose a new approach of target-centered range-only mapping based on RGB-D measurements. Different from the traditional map representation where the 3D locations of the feature points are stored in a global reference coordinate frame, we store the distances from the target to the environmental feature points only. It thus results in a coordinate-free target-centered range-only map. When the target is occluded, it can still be localized accurately based on the map. The proposed approach is verified by both synthetic and real datasets. The experimental results demonstrate the superior performance of our proposed approach to the state-of-the-art RGB-D ORB-SLAM3 method.