Abstract: A feasible approach of secant slopes was proposed to determine the safety state of plastic landslides, which was based on the typical accumulation displacement-time curve of plastic landslides. The secant slope-time (S_t-t) curve was used to determine the stages of landslide deformation. If the S_t-t curve was negative and the curve descended steeply, the landslide was judged in the initial deformation stage. The landslide was in the constant deformation stage if the curve's secant was negative and the curve kept falling slowly; if the curve's secant was from negative to positive and its upward trend was determined, the time point with zero slope could be defined as the critical point of the transformation for landslides from the constant speed deformation stage to accelerated deformation stage; the ratio of the current secant slope to the secant slope of the near five periods could be used to determine the time point from the accelerated deformation stage to rapid deformation stage. If the ratios were greater than 2.0 for the first time and 1.0 for the second time, the landslide entered the rapid deformation stage. According to the example verification, the feasibility and reasonability of the above method were proved.