Abstract: When flying in dynamic environment, fixed-wing UAV (unmanned aerial vehicle) following the reference path will collide with unknown obstacles. To solve the problems, a real-time obstacle avoidance path following algorithm is proposed. Firstly, 3-D Pythagorean hodograph (PH) is adopted as a reference flight path for its continuous curvature and smoothness, which is able to meet the kinematical constraint of fixed-wing UAV and is more suitable for path following. Then, sufficient conditions of stably following a parameterized path in three dimensions are given. A nonlinear model predictive controller is designed for 3-D PH path following, in which the form of objective function is modified to make the UAV timely avoid large unknown static and moving obstacles and fly towards the target after obstacle avoidance during path following. Finally, MATLAB based simulation experiments are performed to demonstrate the effectiveness of the algorithm.