Due to the unique structural property of the structural clay of Zhanjiang group, the bearing capacity of pile is complex in the clay. If the traditional pile foundation design calculation theory and method are applied to calculate the bearing capacity of the pile foundation, the deviation between the theoretical calculation value and the measured value of the pile is more or less. Based on direct shear test and unconfined compression strength test of Zhanjiang group structural clay, as well as the shear displacement method, the model of softening of pile-clay interface is put forward. When the pile-soil interface is in the elastic stage, the side friction of pile increases linearly with displacement. In the stage of softening, the side friction of pile decreases linearly with the displacement. The reduction rate is related to the soil sensitivity and normal pressure of the pile-soil interface. In the stage of sliding, the side friction of pile can not change with the displacement. Therefore, the displacement and axial force analysis of single pile under the vertical load in Zhanjiang group structural clay are derived according to the proposed model. Furthermore, the calculation and test results are compared and the feasibility of the proposed method is verified.