Lateral stress relaxation is one of the important rheological characteristics of soil under confined compression condition. Confined rheological tests are carried out on the revamped consolidation apparatus. The results show that the lateral stress relaxation curves and the vertical creep curves of soil appear synchronous process which consists of unsteady

steady and sudden change stages under confined compression with constant load. Then the mechanism of lateral stress relaxation is analyzed based on the test curves by using the theory of structural grain directional array and the strength theory of soil. Some conclusions can be achieved through mechanism analysis that the three stages relaxation characteristics of lateral stress are induced by the interaction between compaction recovery and shear fracture of soil structure; and the limit state of lateral stress in stable relaxation stage is controlled by the shear strength of soil. In addition

the relaxation extent of lateral stress in higher stress level is more remarkable than in lower stress level. These conclusions will be favorable for further study of the relation between long-term bearing capacity and settlement of ground.