Taiwan is located in the seismic zone of the Pacific Ring of Fire and typhoon pathways, so that its geological and hydrological environment unique and prone to disasters such as landslides, mudslides, and debris flows are very frequent. Consequently, Taiwan has implemented high-standard disaster prevention and control measures, complemented by software-based strategies. Among these, the rainfall-triggered landslide warning threshold plays a crucial role. Moreover, earthquakes have a significant impact on slope stability, leading to internal deformation and reduced strength. Therefore, the post-earthquake rainfall warning threshold should be set lower than that before the earthquake. The current adjustment method relies mainly on historical disaster events and empirical data, lacking a physically-based model. In this study, the numerical analysis is utilized to analyze variations in internal slope strength under different seismic scales for various slope gradients and soil properties. The aim is to establish an adaptive mechanism for adjusting rainfall warning thresholds under different scenarios. The research indicates that slopes composed of medium dense sandy soil exhibit the most significant strength reduction post-earthquake, suggesting a recommended adjustment of the warning threshold by more than 20% and potentially requiring more than six months to recover the strength.