The invention provides a soil anisotropic mechanical property research idea and method from the microscopic point of view by developing a horizontal loading program and a micro-mechanical parameter monitoring program through a particle discrete element unit method. The method includes the steps of (1) obtaining stress-strain curves and related mechanical parameters of cohesionless soil according to laboratory tests; (2) establishing a plane biaxial specimen to simulate soil original anisotropy according to micromechanics parameters determined in a particle flow biaxial test in step (1); (3) establishing a plane biaxial specimen to simulate soil stress anisotropy according to the micromechanics parameters determined in the particle flow biaxial test in step (1); (4) comparing and analyzing expression rules of soil original anisotropy and the soil stress anisotropy to obtain the difference and relation between the two kinds of anisotropy and finding out microscopic mechanisms of the production of the two kinds of soil anisotropy. The soil anisotropic mechanical property research idea and method provides a new idea for the cohesionless soil anisotropic mechanical property numerical simulation by using the advantages of the particle flow discrete element method in the research of the particulate material microstructure.