Wireless sensor network has broad applications in target tracking and locating, especially fit for military detection or guard. By establishing a tracking cluster, this article proposes a Tracking Cluster Rekeying Protocol (TCRP). Sensors can locate the moving object in the monitored area based on given sensitivity and form a tracking cluster around it. This tracking cluster can follow the target logically, process detected data and report to the sink node, and thus achieve the tracking function. To improve its security and applicability, cluster session key is used for data exchange. And pre-established encrypt links are used to guarantee the whole system's security.