Abstract: In this paper, the expression of the moment of inertia of the specimen section that changes with the shear displacement is derived first, and then the relationship curve between the normalized maximum and minimum normal stresses on the specimen shear plane and the shear displacement is calculated. It is found that the two normal stresses and their differences increase with the increase of the shear displacement, and decrease with the increase of the diameter of the specimen shear plane or the shear side length. When the diameter and side length of the shear plane of the sample are the same, the normal stress distribution on the shear plane of the circular sample should be more uniform. When the shear plane of the specimen is rectangular, the normal stress distribution on the shear plane along the long side of the rectangle is more uniform than that along the short side. It is recommended to use the large sample with circular shear surface for direct shear test.