Abstract: The actuator saturation tends to occur in the yaw movement of a quad-rotor UAV (unmanned air vehicle), especially in the presence of disturbances, which results from the fact that yaw movement is much weaker than pitch movement and roll movement. In order to address this problem, a multi-mode proportional-intergral-differential (MMPID) anti-windup controller is proposed from the view point of practical engineering, to prevent actuator saturation in the yaw movement, and the favorable yaw control performance can be guaranteed in the case of external disturbances. Firstly, MMPID method as yaw controller is designed based on the dynamic model of the quad-rotor. MMPID controller has multi-mode characteristic and carries out desaturation strategy under certain conditions. Then, the yaw control system based on MMPID method is proved to be stable using Lyapunov theory. Finally, comparative experiments based on quad-rotor simulation show that MMPID method has significant advantages than variable-structure PID method with respect to the anti-windup performance and yaw control performance. In additon, the validity and robustness of MMPID yaw controller are verified via quad-rotor prototype experiments. Experimental results indicate that the yaw control system based on MMPID method has nearly no saturation with yaw angle error limited to ± 0.05rad indoors without disturbances. It spends 10s to exit saturation with yaw angle error limited to ± 0.08rad indoors with disturbances. It spends 5s to exit saturation with yaw angle error limited to ± 0.13rad outdoors. MMPID controller can effectively prevent actuator saturation in the yaw movement with accurate yaw control and strong robustness for a quad-rotor UAV.