This paper presents a novel solution for the posture control and ride comfort between the proposed wheel-legged robot (four wheel-legged robot (FWLR)) and the unstructured terrain by means of an actively passively transformable suspension system. Unlike most traditional robots, each leg of FWLR is independent of each other with a spring-damping system (passive system) is connected in series with an actuator (active system), so the posture control and ride comfort in complex terrain can be realized by the combination between active and passive systems. To verify the performance of posture control in complex terrain, a prototype and complex terrain are established first, then a posture control model, algorithm, and controller considering the suspension system are proposed and verified by the comparison between co-simulation and experiment, the results showed that the pitch angle and roll angles in complex terrain can be controlled. To show the impact of the actively passively transformable suspension system on ride comfort (vibration isolation performance), different dynamic models with different degree-of–freedom (DOF) are established, the co-simulation results showed that the passive system and active posture control system can also effectively improve the ride comfort of FWLR in complex terrain. The research results of this paper have important reference significance and practical value for enriching and developing the mechanism design and theoretical research of wheel-legged robot and promoting the engineering application of all-terrain robot.