In this paper, the motion coordination algorithm of mobile agents in active sensor network is proposed to track the dynamic boundary for environmental monitoring. While most of dynamic boundary tracking algorithms in the literature were studied under the assumption that the boundary and/or its evolving rate is known a priori, the proposed algorithm is assumed that the individual active agent can measure the state of environment locally without any information of the boundary. When the boundary is evolving dynamically, the formation of active agents is designed to achieve two objectives. One is to track boundary layer based on the measured information and a small deviation. The other is to maintain a uniform distance between adjacent agents. The algorithm structure based on a state diagram is proposed to achieve these two objectives. Finally, it will be shown in the simulations that all given agents converge to a desired boundary layer and maintain a formation along the boundary. (e.g., a circle, an ellipse, a triangle and a rectangle)