Abstract: Laboratory tests were performed on red bed soft rocks. The results show that shear strength parameters from a shear test and a triaxial compressive test are not consistent, and that friction angle changes with a regularity of continuous decreasing from the shear of intact rock to the triaxial compression of rock mass to the shear of structural planes. From the above experimental results, reasons for the difference between the three tests were analyzed respectively based on the strength theory of rock and the methods of laboratory tests. Relationships between the shear strength parameters of rock mass from a triaxial test and the shear strength parameters of structural planes were set up. In addition, the principles of determining the shear strength parameters of rock mass were put forward.