As the application environment of retaining wall is complex,it becomes important to calculate the soil pressure reasonably.About the calculation to the soil pressure of clay,Rankine’s theory about soil pressure does not consider the friction between soil mass and retaining wall,and Coulomb’s theory about soil pressure can only be used to calculate the non-cohesive soil,In order to consider the effect of friction and cohesion,the theory about equivalent internal frication angle was put forward.Three kinds of calculation methods were listed to calculate the equivalent internal frication angle,and the most reasonable calculation method was chosen through analysis.On this basis,a new calculation formula for equivalent internal frication angle was proposed by considering uniform load and water level behind the wall.