<p>Evaluating the stability of slopes under rainfall conditions is essential for preventing geohazards caused by slope failures. A key factor affecting the slope stability is the water-induced weakening of soft rocks during rainfall infiltration. The varying porosities of rocks result in different water content under saturated conditions, leading to varying degrees of weakening in their physical and mechanical properties. The heterogeneity of rocks is also essential in determining slope stability during rainfall. In this study, FISH functions in FLAC^3D were utilized to investigate the impact of different porosities and heterogeneity levels on the peak strength of sandstone during uniaxial compression tests. This methodology was subsequently extended to model stability changes in a two-dimensional rock slope subjected to heavy rainfall. A comparative analysis was conducted between two approaches: one that simplifies the physical and mechanical parameters of the slope model into only dry and saturated states during rainfall infiltration, and another that dynamically adjusts these parameters in real-time based on varying saturation levels. The results demonstrated that the latter approach is more reliable for analyzing slope stability under heavy rainfall conditions. Further, a three-dimensional slope model was employed to validate this approach. The factor of safety (FoS) of the slope was determined to be 1.09 after two days of rainfall, which closely aligns with the observed slope failure in reality. The outcome highlights the importance of accounting for the heterogeneity of weathered layers in rock slopes and dynamically adjusting physical and mechanical parameters according to rock porosity and saturation levels.</p>