Energy dissipation equation is obtained from the virtual work principle. By assuming different initiative sliding directions, methods of upper bound solution and classical solution for rock wedge stability are deduced. The computational formulas of safety factor indicate that the safety factor of wedge is depended on the areas of sliding surfaces and the volume of the wedge rather than the shape of the slope when the attitude and strength of discontinuities is given. Therefore the current wedge method can be improved to overcome the weakness that the current method is inapplicable when the slope shape is greatly rugged and the wedge is surrounded by more than three discontinuities. As an example, the improved method is applied in the left bank slope of Baihetan hydropower station. The calculation results obtained by the improved methods is more reasonable and credible.