Autonomous planning of UAVs(Unmanned Aerial Vehicles) in complicated threat environment has an important practical significance. Based on Pythagorean Hodograph (PH) curve flyable trajectory, the online obstacles avoidance algorithm is studied. For simplification, the three-dimensional collision avoidance problem is decomposed to the horizontal and vertical planes. According to the principle of velocity obstacles, the two-dimensional obstacle avoidance method based on PH curve is given. Simulations on multiple UAVs obstacle avoidance in complicated dynamic threat environment test the validity and the practicability of the algorithm presented in this paper.