Abstract: A triaxial test is a common method for studying the shear strength of soil. This test simulates actual engineering conditions; therefore, the evolution law of the shear band is universal. An in-depth study of this law will help reveal the shear failure characteristics of soil and promote the development of soil mechanics, geological disasters, and other disciplines. To implement directional research on the local deformation of soil samples and optimize the deformation measurement accuracy of soil samples, this paper combined digital image measurements with a triaxial test and used photogrammetry technology to improve this test without a large-scale transformation of the instrument. Taking the red clay soil in Guilin as the research object, the stress and deformation of soil samples on the entire surface were monitored throughout the test, and the accurate deformation data of 136 marked targets on the surface were collected. Based on these data, a three-dimensional reconstruction model of the soil samples was restored to study the overall and local stress–strain relationships, whereas the local strain characteristics were intensively analyzed. Depending on the deformation characteristics of the soil samples during the shear zone development, the evolution law of the shear zone from germination to complete formation was analyzed, and the evolution process was divided into four consecutive stages according to the proposed critical state discrimination method. The results of the study are as follows: (1) The outer and inner regions of the shear band on the soil sample surface have completely different deformation characteristics. In the outer region, the deformation is uniform, and the deformation extent tends to be stable. In the inner region, the deformation is nonuniform, and the deformation extent is continuously intensified. (2) According to the local strain difference between different regions on the surface of the soil samples, the critical state of shear band formation is determined: initial formation, stop expansion, and complete penetration. Moreover, a method is proposed to discriminate the stage of shear band development based on the localized strain on the sample surface. (3) Based on this method, the evolution process of the shear band is divided into four stages: compression deformation, surface expansion, extension penetration, and relative slip, and the different deformation laws and failure mechanisms of each stage are analyzed. The triaxial compression test of soil modified by the photogrammetry technique can obtain more accurate local intrinsic relationships and deformation characteristics, providing an accurate and economical research solution to investigate strain localization phenomenon.