Abstract: Through the series model tests of single pile, raft and single pile with cap, the conclusion that the influencing depth of additional stress is determined by the dimension of the raft and the characteristic of the soil is drawn; the strengthening or weakening action of the cap is analyzed, the cap can strengthen the pile bearing capacity of tip and weaken the pile bearing capacity of skin; considering of the changing of additional stress field, the nonlinear behavior of pile and soil is depicted and the failure pattern is concluded: firstly, the single pile with cap gets into the nonlinear state in the bearing progress, then, the net cap gets into the nonlinear state, all this is determined by the progress of the soil plastically deforming.