This paper proposes an improved A* algorithm for 3D spatial path planning. Considering the complexity of UAV flight environment in low-altitude penetration, the algorithm takes the flight altitude and flight length as the weight factors to search the optimal flight path between two waypoints. The variable-step-size search is used to improve the search efficiency of A* algorithm, and the path evaluation function with variable weight is designed to improve the optimization effect. Meanwhile, to satisfy the UAV performance constraints, such as pitch angle, yaw angle and radius of turn, a series of optimization algorithms are proposed to obtain the final flyable path. The simulation results show that the proposed method can provide more satisfactory path-planning for UAVs．