Abstract: This paper analyzes the variation law of Mohr's stress circle during the shear process of normal consolidated soil in direct shear test and triaxial test. The stress paths of direct shear test and triaxial test during the shear process are studied, which show that the stress path directions of direct shear test and triaxial test are different. According to the presence of the intersection point of two non-parallel straight lines in the same plane, The intersection is not only the vertex of the Moire circle destroyed by the direct shear test but also the vertex of the Moire circle destroyed by the triaxial test. The two normal stresses corresponding to the intersection point in direct shear test and triaxial test are found. Based on the equality of intersection point stress and the relationship between the two normal stresses, And the conversion of coordinate system between direct shear test and triaxial test, a univariate linear equation about normal stress as a variable is theoretically derived. When the normal stress variable of the equation holds for any value, both coefficients of the equation are zero, and then the relationship of shear strength index between the direct shear test and the triaxial test is obtained. Finally direct shear tests and triaxial tests on a certain type of soil samples are conducted. The test results show that the theoretical values are basically consistent with the measured results.