Wireless sensor networks (WSNs) can be used in many applications. Object tracking is one of the most important applications. There are many researches about tracking individual object but few researches on tracking continuous object. Continuous object affects a large number of nodes so needs more communication and cooperation to track it comparing with individual object. In this paper we propose an energy-efficiency continuous object tracking (ECOT) protocol. In ECOT, a sensor node identify itself whether a boundary node or not via automatically adjusting its sensing range instead of communicating with neighboring nodes since radio communication which consumes much more energy than adjusting sensing range. Furthermore just a subset of boundary nodes will send data back to sink node with a small-size message by a representative node selection mechanism. Finally the sink node can estimate the object's position and shape by locating object mechanism. The analysis and simulation results demonstrate that ECOT outperform than previous work and achieves high fidelity and significantly reduces the amount of communication messages.