Abstract: To accurately evaluate the ultimate bearing capacity of rectangular footings on inhomogeneous soil under combined loading is very important in engineering.Based on limit-equilibrium theory and yielding criteria of Mohr-Coulomb,the equilibrium equations can be equivalently transformed to a functional extremization,which boundary values are undetermined,by using the variational method.After introducing the boundary conditions to the functional extremization,the function of the 3-D sliding surface and the failure envelope of rectangular footings on inhomogeneous soil under combined loading are achieved.The effects of the friction angle of soil φ_i,the cohesive force c_i,the ration of L/B,the shear strength ration c_i/c_i+1 and the fluctuation of groundwater level on the ultimate bearing capacity of rectangular footings on inhomogeneous soil foundation and the failure envelop are studied.