In order to study the long-term stability of slope engineering. Whereas the engineering construction in red beds area is increasing. Due to the special engineering geological characteristics of red beds, the red beds soft rock slope is a gradual "aging" process in its whole life. After 5 to 10 years of operation, excavated slopes often appear various serious geological diseases and safety risks. However, the traditional limit equilibrium method is often used to calculate the stability of slope in the early stage, but it is not. According to the characteristics of red beds, the method can not predict the long-term stability and time-dependent deformation characteristics of slope, and can not guide engineering design to formulate economic, safe and reliable targeted pre-reinforcement measures, thus leaving a greater hidden danger for the long-term safe operation in the future. In this paper, taking the 100m-grade deep excavation cutting high slope engineering of a fast-track project in southwest red beds mountainous area as an example, a complex numerical model including special joint and crack elements is established by FLAC3D. The potential deformation range, potential sliding surface location and depth of the slope are calculated and obtained. The creep of the slope at the completion of the first excavation, 20 years without additional support engineering and additional support workers are compared and analyzed. After 20 years of creep, the long-term stability, displacement and stress changes under these three conditions are analyzed. The results show that: Creep, stress relaxation, softening failure and other complex deformation behaviors of high slopes in red beds will occur under long-term service environment. They are ubiquitous in practical projects. The time-dependent deformation of high slopes in red beds should be paid great attention to in later supporting schemes. The results of this paper have been successfully applied to the final engineering design, filling in the blank of the traditional statistical calculation method which is difficult to analyze and solve such problems, in order to provide some reference for the future research and reinforcement design of the time-dependent deformation characteristics of such high soft rock slopes up to 100m magnitude in the red bed area.