Abstract: Based on the careful analysis of the spatial mobilized plane (SMP) constitutive model, it is found that the model is not suitable to the lower stress level and cannot consider the progressive failure process for soil because of its assumption of the linear relation between the stress ratio (the ratio of shear stress to normal stress) and strain ratio (the ratio of normal strain to shear strain) on the SMP. A modified model is presented by assuming that the relation between the stress ratio and shear strain on the SMP is hyperbolic. A comparison between the calculated values of the modified model and the measured values from triaxial tests for sand and clay shows that the modified model is suitable to the overall change extent of stress and strain, and it has the higher precision than the original model as a whole. The constitutive parameters in the modified model can be easily determined by triaxial drained tests.