For the bearings-only tracking problem, an observer's maneuver can enhance the accuracy of estimation. However, it may be idealistic to expect the observer to move in the free space during reconnaissance missions, because threats in the motion space can restrict the motion space of the observer. In this study, considering the constraint of threats avoidance on the motion space of the observer, an optimal maneuver strategy was proposed. Firstly, the adaptive cubature Kalman Filter method was established to improve the robustness and accuracy of state estimation. Then the finite horizon Markov Decision VProcess approach was improved to generate the optimal maneuver policy, in which the quantization method was responsible for discretizing the process and providing the transition matrix for Markov Decision Process, and the novel reward function was proposed as the criterion for optimization. Finally, the framework of the method was established. The numerical results verified the feasibility and advantage of the proposed method by comparing them with state of the art research in the field.KEYWORDS bearing-only tracking, cubature Kalman filter, Markov decision process, maneuver strategy, robustness ---