The shear strength of soil is an important mechanical parameter for slope stability analysis. The large scale direct shear test is an effective method to obtain the shear strength index of coarse grain soil; but it is difficult to excavate massive undisturbed soil. The undisturbed sample was excavated in-situ

the laboratory small scale direct shear tests were carried out on undisturbed sample and compacted sample for comparison. It is approved that the shear strength indices of undisturbed sample could be replaced by the test results of compacted sample when the density and water content of compacted soil could be controlled effectually. The results of small scale shear test of undisturbed soil and large scale shear test of compacted soil were comparatively analyzed. The shear strength of residual soil achieved from small scale direct shear test is more slightly high than that of test results from large scale direct shear test under the same pressure. The research results show that the shear strength indices of small scale direct shear test is slightly bigger than that of slope take place slide in-situ. It is suggested that the peak value strength and remaining strength achieved from large scale direct shear test should be selected for slope stability analysis when the density and water content of compacted soil could be controlled effectually; it is more reasonable to calculate slope stability by this method.