In order to clarify the failure mechanism and ultimate bearing capacity calculation method of layered sand foundation under strip foundation, the finite element plastic limit analysis method is used to calculate and analyze the layered sand foundation, and the calculation results are compared with the existing test data and the calculation results of semi empirical formula, so as to verify the effectiveness of numerical calculation method and the deficiency of semi empirical formula. On this basis, multivariable analysis and calculation are carried out for layered sand foundation. Combined with the theoretical analysis method, the simplified calculation method of foundation ultimate bearing capacity corresponding to three failure modes, boundary and transition failure modes of foundation under strip foundation is given. The results show that according to the failure mechanism of the foundation, the failure forms of the foundation can be divided into punching shear failure, transition failure and general shear failure. The simplified calculation method of the ultimate bearing capacity of the foundation corresponding to the transition failure mode can be used as a reference for specific engineering design.