Abstract: The formation and development of shear zone is the key factor affecting the deformation and damage of soil. We use numerical simulation test method to study the formation and evolutionary mechanism of shear zone in direct shear test for sand. In the test, the sample was marked by the strip method to observe the process of deformation and development, to record the shear stress and shear displacement, and to analyze the stress-strain characteristics of the sample. Finally, the statistical analysis is made on the average rotation, coordination number, porosity change rate and other parameters of the sample from a mesoscopic angle to reveal the mesoscopic deformation law of the sample. The research shows that the formation of sand shear zone is a gradual process, showing the characteristics of progressive failure. During the process of shearing, the particle sample will be deformed locally and form a long and narrow shear zone. The thickness of the shear zone is directly affected by the average particle size and the frictional coefficient of the particles. When the particle size is 1.25 times the average particle size and the frictional coefficient of particle is 0.5, the thickness of the shear band reaches a maximum value. In addition, the strong force chain in the shear zone region is denser, and the time of peak is most significant by analyzing the optical density and area ratio of the strong chain (the average optical density is about 1.4, and the area ratio of the strong chain is about 52%), which indicates the structural changes of the force chain network are synchronous with the formation and evolution of the shear zone. At the same time, it is found that the mesoscopic parameters such as the average rotation amount, the average coordination number and the porosity change rate of the internal and external particle systems in the shear zone are significantly different, which reflects that the shear zone is more strongly affected by external loads.