For a wall-climbing robot, solution of the reasonable values of adhesion force in different states is studied. Firstly, a biped-wheel hybrid locomotion mechanism including closed loop constraint is analyzed and split into open chain mechanisms according to motion equivalence principle. Dynamic model of the open chain mechanism is built using Newton- Euler method. Based on the dynamic model and the critical conditions for motion failure as constraint function, the adhesion force model is built for the wall-climbing robot moving on arbitrarily inclined surface. Then, the reasonable values of adhesion force in different states can be obtained. The simulation and experiments show that the parameters obtained by the force model can ensure adhesion safety. Therefore, the model constructed is reasonable and can provide theoretical basis for reasonable control of adhesion force in various motion states.