A rigid flexible coupled dynamic model of a special vehicle is established with flexibility of the gun tube being considered. Coupling effects on the responses of gun tube are analyzed by comparison with the rigid body model. Then the limit height of the vehicle passing through the vertical boss obstacle road is calculated，and the driving dynamics of the rigid flexible coupled special vehicle under different obstacle conditions is investigated. Furthermore， the suspension system is optimized regarding the suspension stiffness and damping， with the vertical acceleration of the vehicle mass center being the optimization objective. The results indicate that， it is important for coupled modelling to take into account flexibility of gun tube for accurate prediction of vehicle responses， as notable effects on gun's vertical and horizontal displacements， velocities and angular velocity are induced by the flexibility of gun tube. Low vehicle speed and small obstacle height are beneficial to reducing vibration， and the gun tube vertically vibrates more seriously than the vehicle body in different driving conditions. Also， impacts on the vehicle can be reduced for obstacle crossing with single-sided wheel， while vehicle stability can be improved and gun tube's vibration can be weakened for obstacle crossing with two-sided wheels. After optimization of the suspension system， vertical accelerations of both the vehicle and gun tube are notably reduced with the ride comfort being improved.