Abstract: Based on unified strength theory,multi-triangle failure mechanism was considered in the framework of upper-bound method of the plastic limit analysis theory,some basic formulae were deduced,the programme for calculating earth pressure was compiled,and the earth pressure formula that considered the effect of intermediate principal stress was obtained.A series of results could be presented by means of changing the parameter of unified strength theory.These results were compared with other results,and the calculating method could be used to study the effect of the strength theory.These solutions were rigorous upper-bound ones with respect to the solutions for an associated flow rule of Coulomb material.