Abstract: The critical strength of landslides is important for their researches, and how to evaluate it accurately has been the focus and nodus. Conducting ring shear tests on slip soils can obtain the peak and residual strength parameters, and it is helpful to reveal the shearing properties of the soils. Thus, the ring shear tests on the slip soils sampled from Huangtupo landslide are carried out, and two normal stress partitions (namely high- and low-stress subareas) are determined with the boundary stress value of 180 kPa. Then, the soil strengths in this two stress partitions are studied. As a result, the post-peak strength-softening mechanism dominated by the decrease of cohesion (decreased by 44.8% in the high-stress subarea, and 93.8% in the low-stress subarea) and the constant of frictional angle (the difference between peak and residual frictional angles in the high-stress subarea is 0.136°, but 0.468° in the low-stress subarea) is illuminated for the slip soils. By employing a weakening coefficient of cohesion, the back analysis method is adopted to study the critical strength of the reservoir landslide under different working conditions. Eventually, by considering the variation of slope stress and the softening-properties of the soils, the reasons why the critical strength is related with the working conditions are discussed, and their enlightening significance and engineering significance are also expounded.