In light of geo-hazard control in the reservoir area of Three Gorges Project in China,the calculation of critical height of rock-soil elastoplastic area before anti-slide pile on declining sliding surface is carried out with limit equilibrium theory of space wedge block. Firstly,the usual methods of calculating rock-socketed length of anti-slide pile are described,such as engineering geological method and strength controlling method. Secondly,two theories,elastic and elastoplastic theories used to calculati...