The flexibility and cost-effectiveness of unmanned aerial vehicles (UAVs) in a wide range of scenarios have made them indispensable tools for mobile communications. However, UAV communications are also facing increasing security threats, especially in open spaces where they are vulnerable to potential eavesdroppers. In this paper, we employ an extra UAV as a cooperative jammer to enhance UAV-to-ground communication security by sending artificial noise to effectively resist passive eavesdroppers. Unlike previous work, we focus on the dynamic scenario where the source UAV and the ground user move simultaneously. In this scenario, we propose a joint optimization algorithm of trajectory and power for the jamming UAV to obtain the optimal jamming strategy. Due to the absence of eavesdroppers’ locations, the goal of the optimization is to maximize a secure region. Numerical results demonstrate that our optimization algorithm can significantly enlarge the secure region compared to randomly selecting a position of the jamming UAV.