Abstract: In the process of direct shear test, the shear area of the specimen is continuously reduced as the shear displacement increases. If calculated with a constant shear area, the resulting shear stress is not correct. In order to overcome the above shortcomings, this research developed an expression for the shear stress under different shear displacements through theoretical derivation. The study shows that the shear stress obtained after the correction proposed here is greater than the shear stress from the constant shear area method. The difference increases with the increase of the shear displacement, while it becomes smaller with the increase of the diameter or side length of the shear surface. When the diameter or shear side length of the specimen is in the range from 6.18 cm to 50 cm and the shear displacement is in between 2 mm and 60 mm, the cohesion of the soil obtained in this paper increases by 4.9%~14.6%, and the angle of internal friction increases by 3.9%~13.4%. It is recommended to use a specimen with a large shear area for direct shear test.