After jacked pile is driven into the soft clay, a bonding layer would form a around pile body, it enlarges the pile side area and increases its bearing capacity effectively. At present, there are few reports about it. In this paper, the formation mechanism of a bonding layer was studied by theoretical analysis and field test. The results show that the soil particles adhering to a solid surface is an interface phenomenon, the adhesive force is related to soil moisture content, particle size and solid interface conditions. According to the theoretical analysis the bonding layer is the adhesion of tiny colloidal particles on pile surface and is the result of a disturbed soil becomes plastic body from plastic flow after water film disappears. Field test verified the theory and confirmed that the bonding layer was a pile-soil interaction, in the same clay, only the pile with coarse surface has the bonding layer. Finally, a microscopic test of soil samples was carried out, further explaining the formation mechanism of bonding layer.