The thermal stress caused by the difference of temperature in rock slope is an important factor influencing the process of weathering. By using Finite Element Method (FEM), the characteristics of alternating thermal stress field are analyzed, and the criterion of fatigue failure caused by the thermal stress is proposed As an example, the formation mechanism of the concave cavity in the dangerous rock mass in the Chongqing area is analyzed The thermal stress caused by the difference of temperature in rock slope is an important factor influencing the process of weathering. By using Finite Element Method (FEM), the characteristics of alternating thermal stress field are analyzed, and the criterion of fatigue failure caused by the thermal stress is proposed As an example, the formation mechanism of the concave cavity in the dangerous rock mass in the Chongqing area is analyzed