Abstract: It was important to evaluate the ultimate bearing capacity of rectangular footings on inhomogeneous foundation under combined loading.Based on the limit equilibrium theory and the yielding criteria of Mohr-Coulomb,the equilibrium equation could be equivalently transformed to the format of functional extremum with undetermined boundary values by using the variational principle.The function of the 3D sliding surface and the failure envelope of rectangular footings on inhomogeneous foundation under combined loading was achieved when the boundary conditions were introduced.The effect of the angle of internal friction and the cohesion of soil,the ratio of L/B,the shear strength ratio ci/ci+1 and the fluctuation of groundwater level on the ultimate bearing capacity of rectangular footings on inhomogeneous foundation and the failure envelop were studied.