A sliding mode control law was presented according to the six DOF nonlinear model of pitching channel of hypersonic vehicle generic hypersonic vehicle(GHV). First, for making a good use of sliding mode control theory, the six DOF model was transformed into an affine system through input-output feedback linearization. Then, the structure of sliding mode control law was designed for the affine system. After the structure design of sliding mode control law, the control parameters of sliding mode control law were designed based on genetic algorithm. The design process of sliding mode control parameters takes into account stochastic robustness strategy, which is to make sure that the possibility of loss of flight control stability or robustness achieves least through control parameter optimization based on genetic algorithm, when dealing with parameter uncertainties in stochastic uniform distribution. Simulations show that the method is able to satisfy both the robustness of flight control system and the convergence of parameter optimization process on request.