Abstract: The development of economy in China has resulted in more and more infrastructural constructions such as highways in mountainous areas. As a result, more and more man-made slopes are excavated. The stabilities of rock cut slopes with gentle inclined layer structure are generally adequate under normal circumstances. However, they may generate deformation and damage under particular combination of discontinuities. This paper uses l cut highway slopes with weak discontinuities and long joints as an example to study the failure modes and supporting measures. The study is based on investigation of rock mass structure and analysis of slope deformation phenomenon over a certain scope of the studied area. The paper further adopts a three dimension discrete element method to analyze the failure modes of the cut rock slope. The numerical result is in accordance with the result of engineering geological analog. At last, the supporting measures are advanced in close combination with the characteristics of the slope failure mode. The study results indicate that the slipping and cracking deformation can be generated in gentle inclined layered slopes when the discontinuities can form trailing edge and lateral boundary in the slope body. However, there is one special case. The rotary type slipping and cracking deformation can be generated when there is an angle between the trends of sliding surface and slope surface. It is also found that the stabilities of gentle inclined layered slopes are controlled by the key blocks located in the lower-middle part of the slope. The key block failure can cause a chain type of failures of the upper blocks. Consequently, the deformation controlling measures should focus on the region where the key blocks locate. Supporting measures of the cut rock slope established based on the failure mode analysis can guarantee the safety under both construction and operation conditions, which are shown by monitored results.