Abstract: A method of trajectory planning in threedimensional space was proposed based on Improved Ant Colony Algorithm (ACA). The optimization object function of survival probability was transferred, thus the maximum problem of survival probability and the minimization problem of trajectory length could be integrated into one. The consideration of terrain and the constraints were combined with the searching algorithm to make the trajectory more rational and the searching more efficiently. The simulation analyses show that the method of trajectory planning in this paper is applicable and acceptable.