Abstract: This paper is concerned with the long term stability problem of cut slope. To investigate the decrease of strength and the change of strength index of submerged soils, a series of triaxial shear tests were carried out on both submerged and non submerged cohesive soil specimens.Some laboratory simulation tests were also conducted to illustrate the change of stress path of soil in cut slope induced by rainfall and variation of groundwater level. From above tests, we can obtain that the effective cohesion parameter is almost equal to zero and the angle of shearing resistance is near to that of normally consolidated soils under submerging and the long term geological weathering. The validity of the method of strength indexes determination was confirmed by a practical failure case with total stress method and effective stress method . Based on the experimental results, a simplified, practical and economic method of the determination of strength indexes is proposed.