An improved algorithm for 3D path planning based on Basic Theta* algorithm is proposed.In view of the helicopters flight environment in low altitude, the 3D models of threat sources are established,which make full use of the known information of the environment, and take the flight length and threat cost as influence factors.The algorithm adopts the variable-weight evaluation function to improve search efficiency, and reduces invalid checks of node visibility in the process of route search to improve the speed of calculation.Finally, a flyable route between the start point and the end point is obtained.The simulation results show that the improved Theta* algorithm can obtain better path planning results in a shorter time compared with A* algorithm and Theta* algorithm.