Abstract: The are two factors controlling the deformation and failure of slope. One factor is the stress redistribution caused by excavation or loading; the other is variation of shear strength caused by surface water infiltration or groundwater level. Most of the cut loess slopes are above the groundwater level. After excavation, the stress condition does not change much. If no irrigation exsits, the failure of slope is attributed to reduction of shear strength caused by rainfall. The traditional principle of effective stress is not suitable for the strength formula of Mohr-Coulomb because of the negative pore water pressure in unsaturated soils. The Bishops effective stress principle is also useless for its hard-measuring parameters and immature technology. It is worth discussing to determine the shear strength parameters based on the traditional test. The Jijiayuan loess landslide beside the G206 highway as a cut slope is applied to determine the strength parameters c and using the Bishop inverse analysis. Quick direct shear test is used to get the parameters under different moisture contents. Comparing the test result with the inverse result, it is found that they are almost the same when the moisture content is close to the plastic limit. Given that the plastic limit expresses the states of the soil, and when the moisture content exceeds the value to the plastic state, the strength of the soil decreases obviously, leading to failure, which conforms to the consistency and strength principle of the soil. So it is suggested that the shear strength parameters measured by quick direct shear under plastic limit can be applied to the slope stability assessment.