The intermediate principal stress (σ2) effect is a mechanical property inherent in many rock and soil materials. However, the effect of σ2 is often ignored in slope stability analyses, and its impact on slope stability is seldom investigated. The primary purpose of this study is to thoroughly study and analyze the impact of the σ2 effect on slope stability via numerical method. A detailed numerical analysis using the Finite Difference Code Fast Lagrangian Analysis of Continua (FLAC) and Unified Strength Theory (UST) is conducted. The numerical analysis evaluates the values of the Factor of Safety (FOS) for two types of slopes (plane strain and axisymmetric) using the Strength Reduction Method (SRM), and analyzes the impact of the σ2 effect on slope stability. The study found that the influence of the σ2 effect on slope stability is not sensitive to the values of friction angle φ, cohesion c, and slope height H, but increases with increasing slope angle β values. Furthermore, the stability of the plane strain slope is more affected by the σ2 effect than the axisymmetric slope. The impact of the σ2 effect on the footing capacity and slope stability were compared, and the impact of the σ2 effect on slope stability is significantly lower than its effect on the bearing capacity of footings.The innovation of this paper is to systematically analyze the effect of σ2 on slope with different geometric shapes and soil parameters and reveal the inherent characteristics of σ2 effect on the stability of soil slope.