To investigate the influence mechanism of soil creep on the long-term mechanical properties of group pile foundation in high-fill sites, the load-bearing characteristics and deformation patterns of group pile foundations under varying pile spacings (3D~6D) and pile diameters (1.0~2.0 m) were investigated through a combined approach of laboratory model tests and numerical simulations. To address the overly conservative group effect coefficient for negative skin friction in current design codes, reduction recommendations more aligned with actual working conditions were proposed. The findings reveal that soil creep significantly impacts the load-bearing mechanism of pile foundations. During the later stages of creep, the negative skin friction on the pile side reaches its peak and stabilizes, while the end-bearing resistance gradually becomes the primary load-bearing component, with the bearing capacity of the supporting stratum significantly enhanced. Increasing the pile spacing effectively mitigates the additional effects induced by soil creep. Specifically, when the pile spacing is increased from 3D to 6D, the settlement at the pile top and the maximum incremental axial force are reduced by 53.42% and 25.97%, respectively, thereby significantly improving the overall stability of the group pile foundation. Although increasing the pile diameter can reduce additional settlement, the maximum axial force in the pile body rises notably during the later stages of creep, indicating a complex pile-soil interaction mechanism triggered by soil creep. This implies that merely increasing the pile diameter may introduce adverse effects. Furthermore, the current design codes’ group effect coefficient for negative skin friction adopt an overly conservative and fails to accurately represent the actual conditions of high-fill sites. Based on the experimental and numerical simulation results, a recommended reduction range of 62%~96% is proposed to enhance the rationality of design parameters. The research findings provide a theoretical foundation for the design of pile foundations in high-fill sites and offer valuable insights for engineering practices under similar geological conditions.