This paper presents a simple method to control the motion of a quadruped robot in unknown rough terrain using a full dynamic model. First, using the stiffness control method, the behavior of the four legs is approximated using four 3D spring damper systems. In this way, the dynamic model can be derived in Cartesian space. Based on this model, a control strategy is proposed to preserve the asymptotical stability of the system. In addition, a reflex motion control is introduced to cope with the rotational disturbance of the robot body. Finally, dynamic simulations and experiments of a quadruped walking robot were performed on unknown rough terrain to verify the proposed method.