The construction of the 'Dayangyun' Expressway has generated a large number of engineering landslide disaster chains, mainly due to the lack of consideration of the influence of soil sediment anisotropy and slope geometric characteristics on slope stability, instability risk, and failure characteristics. It is urgent to propose a reasonable geometric optimization design method for slopes to prevent the occurrence of such disasters. This study established a random field model that incorporates rotational anisotropy-related structures of strength parameters. Subsequently, the slope reliability index(beta) was computed to evaluate slope stability. Additionally, failure modes were classified, introducing the shallow failure probability (PL) to assess failure risk. Finally, a comprehensive probability analysis framework with two indexes(the beta and PL) was designed to determine the optimal platform width of the slope(Lopt), and two slope cases were utilized for research and application purposes. The results indicate that rotation angles(theta) and platform width (L) significantly impact slope stability and instability risk. As the theta increases, the beta and PL exhibit S and M shaped trends, respectively. Specifically, the beta and PL display a logarithmic and exponential increasing trend with the increase of the L, respectively, this trend determines the Lopt. The dual-index comprehensive probability analysis framework can be employed to assess slope excavation stability and risk, as well as optimize slope geometry design. The research results can be used to prevent the occurrence of excavation slope disasters.