When structures are built on soft soil, the bearing capacity can be hardly satisfied as the soil has the characteristics of high water ratio, high compressibility and poor shear strength. However, the bearing capacity can be improved through structure itself by taking full advantage of the interaction between the structure and soil. Firstly, an experiment is carried out to compare the measured value of the experiment with that of the Plaxis simulation. The results show that the Hardening-Soil model is able to simulate the bearing capacities of the box and bucket foundation on soft soil. Then, the ultimate bearing capacity of the structure on one layer soil and stratified soil are studied. The influence factors of structure form, structure dimension, parameter of soil and the thickness of the layer are analyzed, and modified formulas of quadrate, circular and strip box and bucket foundation are concluded. Finally, using FEM program Plaxis, the paper establishes a numerical analysis model for the box and bucket foundation of Tianjin harbor breakwater extension project.