Abstract: To investigate the influence of water-bearing miscellaneous fill layers on the stability of tunnel portal slopes after heavy rainfall, the landslide event at the Tangshan Road Tunnel portal was selected as a case study. First, a dedicated geological investigation was conducted to characterize the soil layers within the landslide zone and to determine the extent and depth of the water-enriched area on the portal slope. Then, the stability of the original reinforcement scheme was evaluated using the strength reduction method. Following the principle of water-first slope stabilization, an improved reinforcement scheme combining dewatering and pumping, two-fluid grouting with small pipes, and gentle slope forming was proposed. Subsequently, the shear strength parameters of the grouted soil were determined through laboratory testing and the yield function of the two-fluid composite material. The safety stability coefficients of the portal slope under different groundwater levels were then compared and analyzed. The results show that the fill layer in the landslide area exhibits strong heterogeneity, low bearing capacity, and high permeability. Under heavy rainfall conditions, the safety factor of the original reinforcement scheme fell below the design requirements, consistent with the observed field collapse. When the groundwater level was reduced to 74.5 m under heavy rainfall conditions, the proposed scheme reduced the maximum displacement and shear stress of the portal slope by 64.2% and 20.3%, respectively, compared to the original scheme (with groundwater level at 88 m), while the safety factor increased by 49.5%. Field monitoring confirmed that, after applying the improved reinforcement measures, the surface settlement of the portal slope remained within the specified design limits.