With a first-principles method the energy barrier surface of the dissociative adsorption for hydrogen molecules impinging on a Si(111)(1x1) surface was investigated. A large corrugated energy barrier surface was found and the corrugation of the surface is much larger than that of H-2 on a Cu(100) surface. The energy barrier strongly depends on the orientations of the molecule and the position of H-2 on the surface. The large corrugation of the energy barrier surface and the strong orientation effect are the main cause for the low sticking coefficient of the hydrogen molecule impinging on a Si(111) surface.