Taking as the project background of the gravel filling between pile casings of a bridge in Guinan High-Speed Railway to maintain the perpendicularity of long piles, a research has been conducted on the impact of gravel filling on the load transfer of pile foundation. Based on a simulation test of the filling tightness of gravel porosity, an FLAC-PFC pile foundation load transfer coupling numerical model is established under different levels of filling tightness. The numerical analysis results show that the vertical load is mainly borne by the pile end resistance with a large vertical displacement of the gravel around the pile, which is proportional to the increase of the porosity; meanwhile, the horizontal displacement tends to be small, which has little relationship with the porosity.