To study the obstacle negotiation capacity of reconfigurable wheel-manipulator robots, the combined wheel-manipulator obstacle negotiation process was described, and based on the screw theory, the dynamic formulation of stair-climbing on a slope for reconfigurable wheel-manipulator robots was established by using the virtual mechanism method. Because of the diversity of configuration and the flexibility of motion, the dynamic formulation using conventional approaches is complex, moreover, it is difficult to expansion to multiple modules. Whereas adopting the screw theory, concise and unified description of complex mechanisms can be derived , and the Jacobian matrix is easily obtained by exponential product formula. Three factors affecting the motor torque mainly , such as stair height, arm length and kinematic parameters, were simulated, and the stability in the obstacle-negotiation process was analyzed and the result of stable working range was obtained. The maximum stair-height that the robot can negotiate was achieved by simulation and experiment.