Abstract: In order to study the arching effect of cohesive soil on the calculation of the active earth pressure behind the retaining wall, a new lateral pressure coefficient formula related to the angle of slip surface and the tension crack was first derived and further studied. Then based on the analysis of the soil wedge model, the formula used to calculate the angle of slip surface was deduced with consideration of the soil-wall friction angle and cohesion. At last, based on the analysis of the microelement model and using the static mechanic method, the new vertical mean stress distribution formula was derived. The calculated results of sand and clay soils were in an agreement with the test data. Therefore, it was necessary considering the arching effect of slip surface angle in the calculation of the active earth pressure.