Abstract: Clay liners play a crucial role in preventing leakage and migration of landfill leachate. Non-darcy seepage often appears in clay, with an initial hydraulic gradient. However, the effect of initial hydraulic gradient on the anti-fouling performance of clay liners is not yet clear. Therefore, the influence of initial hydraulic gradient on contaminant migration was studied in this paper. Firstly, the deionized water was used for the clay penetration test under different confining pressure. Secondly, under the confining pressure σ of 200 kpa, Na+ solution was used for permeability test under 4 parallel compacted clay samples with hydraulic gradients of 0, 12.5, 25 and 375, the concentration of Na+ in the leachate was determined regularly. Finally, the concentration of Na+ in the samples was determined in layers. The results show that the initial hydraulic gradient increases with the increase of consolidation pressure. When the hydraulic gradient is 0, 12.5, and 25, there was no osmotic liquid at the seepage outlet and the Na+ migration rule is basically the same, i.e., the molecular diffusion coefficient is almost equal. When the hydraulic gradient was 375, the hydrodynamic dispersion coefficient is much larger than the molecular diffusion coefficient. Therefore, it can be considered that when the hydraulic gradient is less than i0, the leachate is impermeable or small permeable, and the pollutant migration is dominated by molecular diffusion. When the hydraulic gradient is greater than i0, the pollutant migration gradually turns to convection.