With the development of geomechanics and the requirements of many large-scale engineering projects，soil-rock mixtures(SRM) have been regarded as a special type of soil and rock materials which are substantially different with the general soils and rocks in geotechnical engineering. In a large extent，the proportion and distribution of the rock block sizes control the physico-mechanical properties of SRM. Using digital image processing，rock block sizes in SRM samples are identified from the soil matrix. The proportion and distribution of the rock block sizes are then obtained quantitatively. Then the results are used for the sample preparation of the large scale direct shear tests，which cuts a new method for the test study of SRM. According to the results of the large scale direct shear tests，as a whole，the rock block size proportion controls the deformation and fracture mechanism of SRM. The shape of the shear stress versus the horizontal displacement curve and the vertical displacement versus the horizontal displacement curve of the remolded SRM samples are different from the general soils and rocks. With the increase of the rock block proportion，the shear band of SRM increased. When the rock block proportion lied in the range of 25% to 70%，the increment of the internal frication angle increased linearly with the increase of the rock block proportion. And the cohesion of the SRM decreased largely from the soil matrix. When the rock block proportion is large than 30%，however，there is only a little decrease in the cohesion with the increase of the rock block proportion.