Object tracking is an important feature of the ubiquitous society and also a killer application of wireless sensor networks. Nowadays, there are many researches on object tracking in wireless sensor networks under practice, however most of them cannot effectively deal with the trade-off between missing-rate and energy efficiency. In this paper, we propose a dynamic clustering mechanism for object tracking in wireless sensor networks. With forming the cluster dynamically according to the route of moving, the proposed method can not only decrease the missing-rate but can also decrease the energy consumption by reducing the number of nodes that participate in tracking and minimizing the communication cost, thus can enhance the lifetime of the whole sensor networks. The simulation result shows that our proposed method achieves lower energy consumption and lower missing-rate.