To guarantee the detection accuracy and viability of the single observer, a trajectory optimization method of the single observer with the battlefield threats′ constraint was proposed. The threatened factors in battlefield were analyzed, and the quantitative model between the battlefield threats and the trajectory of the observer was obtained. On that basis, considering both the accuracy and the threats extent of the trajectory, the accuracy score function and the threats score function were built, respectively. Then, the course choice function was constructed, and the different trajectories of the observer were evaluated. Simulation results show that the proposed method can avoid the threats effectively and improve the detection accuracy as well; the accuracy and reliability of single-observer bearings-only detection in battlefield can be guaranteed through the method.