Abstract: Almost all the constitutive models for soils are established on the basis of the stress-strain relationship curve of soil samples. In this case, the stress-strain relationship curve reflects the deformation characteristics of a soil sample as a whole, in which the soil sample is considered as a representative element volume. In this paper, by adopting the deformation field (local deformation) over the entire surface of soil samples in triaxial tests, local deformation characteristics can be obtained by measuring the deformation process at the feature points over the surface of the soil samples. It is found that after the shear failure occurs the deformation feature in the shear band is quite different from that out of the shear band. According to the analysis of the occurrence and development of the shear band, the samples apparently exhibit three states during the tests, i.e., pre-failure, in-failure and post-failure, in correspondence to different deformation features. In the pre-failure state, the deformation of the entire sample is approximately uniform, and the global stress-strain curve is representative. In the failure state, failure occurs from a point (or some points) and develops gradually until the shear band cleaves the sample; in addition, the observed deformation for the entire sample may be the combination of deformations in the failure zones and non-failure zones. In the post-failure state, load (stress) does not continue to increase, and the deformation of the top and bottom blocks out of the shear band also does not continue to increase as a rigid body, and at the same time, the deformation is exclusively owing to the blocks of the sample sliding along the shear band. Therefore, it is not appropriate to define the strain of the sample to result from frictional sliding along the shear band. The stress-strain curve of the entire sample reveals structural response of soil, not an elementary response. So we think that the constitutive models for soils should include the stress-strain relationship of soils in the pre-failure state, failure criterion and frictional sliding characteristic along the shear band in post-failure state. The important point of establishing the constitutive model is how to describe the stress-strain relationship of soils in the pre-failure state. The so-called “critical state” is actually a rigid sliding state along the shear failure surface.