Many loess multi-stage slopes have been constructed in gully land consolidation projects on the Chinese Loess Plateau, which are vulnerable to failure induced by unreasonable excavation or rainfall. However, few studies have analyzed the influence of slope shape parameters and their design optimization on the stability of multi-stage slopes. We selected the Nangou catchment as a case study. Based on detailed field investigations, numerical analysis was conducted to reveal the influence of slope shape parameters (single stage slope height h, single stage slope ratio i, and berm width d) and platform setting on displacement, shear strain, plastic zone distribution, and the factor of safety (FS) of the multi-stage slope. Additionally, the sensitivity analysis method was used to compare and analyze the sensitivity of the slope shape parameters to FS. The results show that FS is negatively correlated with h and i and positively correlated with d. The sensitivity order of slope shape parameters to FS is i > d > h. The setting of a platform separates the shallow plastic zone of the slope into two independent parts to prevent larger landslides. In addition, based on the function relationship of FS with slope height and gradient: FS = a(tan theta)(b) (where a and b are functions of the slope height), the design optimization of the multi-stage slope with different slope heights and a comprehensive treatment are proposed. The research results could provide theoretical support for the design and prevention of related risks for excavation slopes in gully land consolidation projects.