Abstract: In order to analyze the coupled lateral-directional dynamics of plane-symmetrical high speed vehicles, the coupled model was built, which involves kinematic coupling and aerodynamic coupling and control coupling. The analysis method for coupled dynamics was proposed, which is based on two parameters: the ratio of rollingmoment coefficient versus sideslip to yawing moment coefficient versus sideslip and the ratio of aileron effect on rolling moment to yawing moment. Due to the cross-control effect, the system has non-minimum phase behavior. The criterion of control departure was deduced, which involves yawing moment coefficient versus sideslip and aileron cross-control effect on yawing moment and rudder control effect on yawing moment. The characteristics of coupled mode and control departure were analyzed. Simulation results show that the method is reasonable.