There has been controversy on the calculation method of soil-water pressure in the design of the supporting structure. There are two methods to calculate soil-water pressure, soil-water pressure calculated separately and soil-water pressure calculated jointly. The theoretical mechanism of the former is very clear and definite, but pore pressure is difficult to measure. The latter can be suitably applied to some engineering, but the theoretical mechanism is not clear and definite. There is a special condition in retaining structures during operation period, temporary load is imposed on the surface of the soil. For the special condition, according to the CU strength envelope the method of determining the soil's state under the condition of undrained condition. It is based on the intrinsic connection between unconsolidated undrained triaxial tests and consolidated undrained triaxial tests. The principal stress and the excess pore pressure are calculated which are coursed by the increase of load, it is based on the CU strength envelope. A new approach is provided on the calculation of soil-water pressure for retaining structures under special conditions.