Obstacle negotiation capability is important to a mobile robot. A wheel-legged robot with symmetrical structure was presented It has four independent wheel-legged articulations which can generate a series of conFigureurations to improve its trafficability. The obstacle negotiation capability of the robot was studied. Its step-climbing process was described, and the geometrical and dynamic model for the process was built. Based on comprehensively considering geometrical and dynamic constraints, the method to work out the maximal step height that the robot can get across was brought forward. Then the method to work out the maximal slope angle was also provided. Finally the experiment to verify the above methods was done. The research mentioned above would provide a theoretical foundation to improve the robot’s adaptability in complicated environments.