By analyzing the stress state of soil around a pile and friction characteristic of the pile-soil interface during the load bearing process of the pile foundation, a calculation model of shear strength of soil based on stress state of soil and calculation model of shear strength of interface based on friction characteristic of pile-soil interface is derived. By comparing the shear strength of soil with that of the interface, the calculating model of ultimate shaft resistance under the influence of the coupling of the two kinds of strength is derived, and the rationality and feasibility of the model is verified by an engineering example in Shanghai, and the distribution law of ultimate shaft resistance of different types of soil with buried depth is analyzed by using the model. The results show that, due to the different relations between the internal friction angle and the lateral pressure coefficient, the ultimate shaft resistance shows different distribution characteristics with buried depth, and in a certain relationship, there is a critical depth beyond which the ultimate shaft resistance is maintained at a stable value or even reduced constantly to zero.